{-# OPTIONS_GHC -Wno-incomplete-uni-patterns   #-}

module GHC.Tc.Solver.Interact (
     solveSimpleGivens,   -- Solves [Ct]
     solveSimpleWanteds   -- Solves Cts
  ) where

import GHC.Prelude
import GHC.Types.Basic ( SwapFlag(..), IntWithInf, intGtLimit )
import GHC.Tc.Solver.Canonical
import GHC.Types.Var.Set

import GHC.Types.Var
import GHC.Tc.Errors.Types
import GHC.Tc.Utils.TcType
import GHC.Builtin.Names ( coercibleTyConKey, heqTyConKey, eqTyConKey, ipClassKey )
import GHC.Tc.Instance.FunDeps
import GHC.Tc.Instance.Family
import GHC.Tc.Instance.Class ( InstanceWhat(..), safeOverlap )

import GHC.Tc.Types.Evidence
import GHC.Utils.Outputable
import GHC.Utils.Panic

import GHC.Tc.Types
import GHC.Tc.Types.Constraint
import GHC.Tc.Types.Origin
import GHC.Tc.Utils.TcMType( promoteMetaTyVarTo )
import GHC.Tc.Solver.Types
import GHC.Tc.Solver.InertSet
import GHC.Tc.Solver.Monad

import GHC.Core
import GHC.Core.Type as Type
import GHC.Core.InstEnv     ( DFunInstType )
import GHC.Core.Class
import GHC.Core.TyCon
import GHC.Core.Predicate
import GHC.Core.Coercion
import GHC.Core.FamInstEnv
import GHC.Core.Unify ( tcUnifyTyWithTFs, ruleMatchTyKiX )
import GHC.Core.Coercion.Axiom ( CoAxBranch (..), CoAxiom (..), TypeEqn, fromBranches
                               , sfInteractInert, sfInteractTop )

import GHC.Types.SrcLoc
import GHC.Types.Var.Env
import GHC.Types.Unique( hasKey )

import GHC.Data.Bag
import GHC.Data.Pair (Pair(..))

import GHC.Utils.Monad ( concatMapM, foldlM )
import GHC.Utils.Misc

import GHC.Driver.Session

import qualified GHC.LanguageExtensions as LangExt

import Data.List( deleteFirstsBy )
import Data.Maybe ( listToMaybe, mapMaybe )
import Data.Function ( on )
import qualified Data.Semigroup as S

import Control.Monad.Trans.Class
import Control.Monad.Trans.Maybe
import Control.Monad

{-
**********************************************************************
*                                                                    *
*                      Main Interaction Solver                       *
*                                                                    *
**********************************************************************

Note [Basic Simplifier Plan]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1. Pick an element from the WorkList if there exists one with depth
   less than our context-stack depth.

2. Run it down the 'stage' pipeline. Stages are:
      - canonicalization
      - inert reactions
      - spontaneous reactions
      - top-level interactions
   Each stage returns a StopOrContinue and may have sideeffected
   the inerts or worklist.

   The threading of the stages is as follows:
      - If (Stop) is returned by a stage then we start again from Step 1.
      - If (ContinueWith ct) is returned by a stage, we feed 'ct' on to
        the next stage in the pipeline.
4. If the element has survived (i.e. ContinueWith x) the last stage
   then we add it in the inerts and jump back to Step 1.

If in Step 1 no such element exists, we have exceeded our context-stack
depth and will simply fail.
-}

solveSimpleGivens :: [Ct] -> TcS ()
solveSimpleGivens :: [Ct] -> TcS ()
solveSimpleGivens [Ct]
givens
  | [Ct] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [Ct]
givens  -- Shortcut for common case
  = () -> TcS ()
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ()
  | Bool
otherwise
  = do { String -> SDoc -> TcS ()
traceTcS String
"solveSimpleGivens {" ([Ct] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Ct]
givens)
       ; [Ct] -> TcS ()
go [Ct]
givens
       ; String -> SDoc -> TcS ()
traceTcS String
"End solveSimpleGivens }" SDoc
forall doc. IsOutput doc => doc
empty }
  where
    go :: [Ct] -> TcS ()
go [Ct]
givens = do { Cts -> TcS ()
solveSimples ([Ct] -> Cts
forall a. [a] -> Bag a
listToBag [Ct]
givens)
                   ; [Ct]
new_givens <- TcS [Ct]
runTcPluginsGiven
                   ; Bool -> TcS () -> TcS ()
forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
when ([Ct] -> Bool
forall (f :: * -> *) a. Foldable f => f a -> Bool
notNull [Ct]
new_givens) (TcS () -> TcS ()) -> TcS () -> TcS ()
forall a b. (a -> b) -> a -> b
$
                     [Ct] -> TcS ()
go [Ct]
new_givens }

solveSimpleWanteds :: Cts -> TcS WantedConstraints
-- The result is not necessarily zonked
solveSimpleWanteds :: Cts -> TcS WantedConstraints
solveSimpleWanteds Cts
simples
  = do { String -> SDoc -> TcS ()
traceTcS String
"solveSimpleWanteds {" (Cts -> SDoc
forall a. Outputable a => a -> SDoc
ppr Cts
simples)
       ; DynFlags
dflags <- TcS DynFlags
forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
       ; (ScDepth
n,WantedConstraints
wc) <- ScDepth
-> IntWithInf
-> WantedConstraints
-> TcS (ScDepth, WantedConstraints)
go ScDepth
1 (DynFlags -> IntWithInf
solverIterations DynFlags
dflags) (WantedConstraints
emptyWC { wc_simple = simples })
       ; String -> SDoc -> TcS ()
traceTcS String
"solveSimpleWanteds end }" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
             [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"iterations =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> ScDepth -> SDoc
forall a. Outputable a => a -> SDoc
ppr ScDepth
n
                  , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"residual =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> WantedConstraints -> SDoc
forall a. Outputable a => a -> SDoc
ppr WantedConstraints
wc ]
       ; WantedConstraints -> TcS WantedConstraints
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return WantedConstraints
wc }
  where
    go :: Int -> IntWithInf -> WantedConstraints -> TcS (Int, WantedConstraints)
    go :: ScDepth
-> IntWithInf
-> WantedConstraints
-> TcS (ScDepth, WantedConstraints)
go ScDepth
n IntWithInf
limit WantedConstraints
wc
      | ScDepth
n ScDepth -> IntWithInf -> Bool
`intGtLimit` IntWithInf
limit
      = TcRnMessage -> TcS (ScDepth, WantedConstraints)
forall a. TcRnMessage -> TcS a
failTcS (TcRnMessage -> TcS (ScDepth, WantedConstraints))
-> TcRnMessage -> TcS (ScDepth, WantedConstraints)
forall a b. (a -> b) -> a -> b
$ Cts -> IntWithInf -> WantedConstraints -> TcRnMessage
TcRnSimplifierTooManyIterations Cts
simples IntWithInf
limit WantedConstraints
wc
     | Cts -> Bool
forall a. Bag a -> Bool
isEmptyBag (WantedConstraints -> Cts
wc_simple WantedConstraints
wc)
     = (ScDepth, WantedConstraints) -> TcS (ScDepth, WantedConstraints)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (ScDepth
n,WantedConstraints
wc)

     | Bool
otherwise
     = do { -- Solve
            WantedConstraints
wc1 <- WantedConstraints -> TcS WantedConstraints
solve_simple_wanteds WantedConstraints
wc

            -- Run plugins
          ; (Bool
rerun_plugin, WantedConstraints
wc2) <- WantedConstraints -> TcS (Bool, WantedConstraints)
runTcPluginsWanted WantedConstraints
wc1

          ; if Bool
rerun_plugin
            then do { String -> SDoc -> TcS ()
traceTcS String
"solveSimple going round again:" (Bool -> SDoc
forall a. Outputable a => a -> SDoc
ppr Bool
rerun_plugin)
                    ; ScDepth
-> IntWithInf
-> WantedConstraints
-> TcS (ScDepth, WantedConstraints)
go (ScDepth
nScDepth -> ScDepth -> ScDepth
forall a. Num a => a -> a -> a
+ScDepth
1) IntWithInf
limit WantedConstraints
wc2 }   -- Loop
            else (ScDepth, WantedConstraints) -> TcS (ScDepth, WantedConstraints)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (ScDepth
n, WantedConstraints
wc2) }           -- Done


solve_simple_wanteds :: WantedConstraints -> TcS WantedConstraints
-- Try solving these constraints
-- Affects the unification state (of course) but not the inert set
-- The result is not necessarily zonked
solve_simple_wanteds :: WantedConstraints -> TcS WantedConstraints
solve_simple_wanteds (WC { wc_simple :: WantedConstraints -> Cts
wc_simple = Cts
simples1, wc_impl :: WantedConstraints -> Bag Implication
wc_impl = Bag Implication
implics1, wc_errors :: WantedConstraints -> Bag DelayedError
wc_errors = Bag DelayedError
errs })
  = TcS WantedConstraints -> TcS WantedConstraints
forall a. TcS a -> TcS a
nestTcS (TcS WantedConstraints -> TcS WantedConstraints)
-> TcS WantedConstraints -> TcS WantedConstraints
forall a b. (a -> b) -> a -> b
$
    do { Cts -> TcS ()
solveSimples Cts
simples1
       ; (Bag Implication
implics2, Cts
unsolved) <- TcS (Bag Implication, Cts)
getUnsolvedInerts
       ; WantedConstraints -> TcS WantedConstraints
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (WC { wc_simple :: Cts
wc_simple = Cts
unsolved
                    , wc_impl :: Bag Implication
wc_impl   = Bag Implication
implics1 Bag Implication -> Bag Implication -> Bag Implication
forall a. Bag a -> Bag a -> Bag a
`unionBags` Bag Implication
implics2
                    , wc_errors :: Bag DelayedError
wc_errors = Bag DelayedError
errs }) }

{- Note [The solveSimpleWanteds loop]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Solving a bunch of simple constraints is done in a loop,
(the 'go' loop of 'solveSimpleWanteds'):
  1. Try to solve them
  2. Try the plugin
  3. If the plugin wants to run again, go back to step 1
-}

-- The main solver loop implements Note [Basic Simplifier Plan]
---------------------------------------------------------------
solveSimples :: Cts -> TcS ()
-- Returns the final InertSet in TcS
-- Has no effect on work-list or residual-implications
-- The constraints are initially examined in left-to-right order

solveSimples :: Cts -> TcS ()
solveSimples Cts
cts
  = {-# SCC "solveSimples" #-}
    do { (WorkList -> WorkList) -> TcS ()
updWorkListTcS (\WorkList
wl -> (Ct -> WorkList -> WorkList) -> WorkList -> Cts -> WorkList
forall a b. (a -> b -> b) -> b -> Bag a -> b
forall (t :: * -> *) a b.
Foldable t =>
(a -> b -> b) -> b -> t a -> b
foldr Ct -> WorkList -> WorkList
extendWorkListCt WorkList
wl Cts
cts)
       ; TcS ()
solve_loop }
  where
    solve_loop :: TcS ()
solve_loop
      = {-# SCC "solve_loop" #-}
        do { Maybe Ct
sel <- TcS (Maybe Ct)
selectNextWorkItem
           ; case Maybe Ct
sel of
              Maybe Ct
Nothing -> () -> TcS ()
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ()
              Just Ct
ct -> do { [(String, SimplifierStage)] -> Ct -> TcS ()
runSolverPipeline [(String, SimplifierStage)]
thePipeline Ct
ct
                            ; TcS ()
solve_loop } }

-- | Extract the (inert) givens and invoke the plugins on them.
-- Remove solved givens from the inert set and emit insolubles, but
-- return new work produced so that 'solveSimpleGivens' can feed it back
-- into the main solver.
runTcPluginsGiven :: TcS [Ct]
runTcPluginsGiven :: TcS [Ct]
runTcPluginsGiven
  = do { [TcPluginSolver]
solvers <- TcS [TcPluginSolver]
getTcPluginSolvers
       ; if [TcPluginSolver] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcPluginSolver]
solvers then [Ct] -> TcS [Ct]
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return [] else
    do { [Ct]
givens <- TcS [Ct]
getInertGivens
       ; if [Ct] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [Ct]
givens then [Ct] -> TcS [Ct]
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return [] else
    do { TcPluginProgress
p <- [TcPluginSolver] -> SplitCts -> TcS TcPluginProgress
runTcPluginSolvers [TcPluginSolver]
solvers ([Ct]
givens,[])
       ; let ([Ct]
solved_givens, [(EvTerm, Ct)]
_) = TcPluginProgress -> ([Ct], [(EvTerm, Ct)])
pluginSolvedCts TcPluginProgress
p
             insols :: [Ct]
insols             = TcPluginProgress -> [Ct]
pluginBadCts TcPluginProgress
p
       ; (InertCans -> InertCans) -> TcS ()
updInertCans ([Ct] -> InertCans -> InertCans
removeInertCts [Ct]
solved_givens)
       ; (Cts -> Cts) -> TcS ()
updInertIrreds (\Cts
irreds -> Cts -> [Ct] -> Cts
extendCtsList Cts
irreds [Ct]
insols)
       ; [Ct] -> TcS [Ct]
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (TcPluginProgress -> [Ct]
pluginNewCts TcPluginProgress
p) } } }

-- | Given a bag of (rewritten, zonked) wanteds, invoke the plugins on
-- them and produce an updated bag of wanteds (possibly with some new
-- work) and a bag of insolubles.  The boolean indicates whether
-- 'solveSimpleWanteds' should feed the updated wanteds back into the
-- main solver.
runTcPluginsWanted :: WantedConstraints -> TcS (Bool, WantedConstraints)
runTcPluginsWanted :: WantedConstraints -> TcS (Bool, WantedConstraints)
runTcPluginsWanted wc :: WantedConstraints
wc@(WC { wc_simple :: WantedConstraints -> Cts
wc_simple = Cts
simples1 })
  | Cts -> Bool
forall a. Bag a -> Bool
isEmptyBag Cts
simples1
  = (Bool, WantedConstraints) -> TcS (Bool, WantedConstraints)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (Bool
False, WantedConstraints
wc)
  | Bool
otherwise
  = do { [TcPluginSolver]
solvers <- TcS [TcPluginSolver]
getTcPluginSolvers
       ; if [TcPluginSolver] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TcPluginSolver]
solvers then (Bool, WantedConstraints) -> TcS (Bool, WantedConstraints)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (Bool
False, WantedConstraints
wc) else

    do { [Ct]
given <- TcS [Ct]
getInertGivens
       ; Cts
wanted <- Cts -> TcS Cts
zonkSimples Cts
simples1    -- Plugin requires zonked inputs
       ; TcPluginProgress
p <- [TcPluginSolver] -> SplitCts -> TcS TcPluginProgress
runTcPluginSolvers [TcPluginSolver]
solvers ([Ct]
given, Cts -> [Ct]
forall a. Bag a -> [a]
bagToList Cts
wanted)
       ; let ([Ct]
_, [(EvTerm, Ct)]
solved_wanted)   = TcPluginProgress -> ([Ct], [(EvTerm, Ct)])
pluginSolvedCts TcPluginProgress
p
             ([Ct]
_, [Ct]
unsolved_wanted) = TcPluginProgress -> SplitCts
pluginInputCts TcPluginProgress
p
             new_wanted :: [Ct]
new_wanted                             = TcPluginProgress -> [Ct]
pluginNewCts TcPluginProgress
p
             insols :: [Ct]
insols                                 = TcPluginProgress -> [Ct]
pluginBadCts TcPluginProgress
p

-- SLPJ: I'm deeply suspicious of this
--       ; updInertCans (removeInertCts $ solved_givens)

       ; ((EvTerm, Ct) -> TcS ()) -> [(EvTerm, Ct)] -> TcS ()
forall (t :: * -> *) (m :: * -> *) a b.
(Foldable t, Monad m) =>
(a -> m b) -> t a -> m ()
mapM_ (EvTerm, Ct) -> TcS ()
setEv [(EvTerm, Ct)]
solved_wanted
       ; (Bool, WantedConstraints) -> TcS (Bool, WantedConstraints)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ( [Ct] -> Bool
forall (f :: * -> *) a. Foldable f => f a -> Bool
notNull (TcPluginProgress -> [Ct]
pluginNewCts TcPluginProgress
p)
                , WantedConstraints
wc { wc_simple = listToBag new_wanted       `andCts`
                                   listToBag unsolved_wanted  `andCts`
                                   listToBag insols } ) } }
  where
    setEv :: (EvTerm,Ct) -> TcS ()
    setEv :: (EvTerm, Ct) -> TcS ()
setEv (EvTerm
ev,Ct
ct) = case Ct -> CtEvidence
ctEvidence Ct
ct of
      CtWanted { ctev_dest :: CtEvidence -> TcEvDest
ctev_dest = TcEvDest
dest } -> TcEvDest -> EvTerm -> TcS ()
setWantedEvTerm TcEvDest
dest EvTerm
ev
      CtEvidence
_ -> String -> TcS ()
forall a. HasCallStack => String -> a
panic String
"runTcPluginsWanted.setEv: attempt to solve non-wanted!"

-- | A pair of (given, wanted) constraints to pass to plugins
type SplitCts  = ([Ct], [Ct])

-- | A solved pair of constraints, with evidence for wanteds
type SolvedCts = ([Ct], [(EvTerm,Ct)])

-- | Represents collections of constraints generated by typechecker
-- plugins
data TcPluginProgress = TcPluginProgress
    { TcPluginProgress -> SplitCts
pluginInputCts  :: SplitCts
      -- ^ Original inputs to the plugins with solved/bad constraints
      -- removed, but otherwise unmodified
    , TcPluginProgress -> ([Ct], [(EvTerm, Ct)])
pluginSolvedCts :: SolvedCts
      -- ^ Constraints solved by plugins
    , TcPluginProgress -> [Ct]
pluginBadCts    :: [Ct]
      -- ^ Constraints reported as insoluble by plugins
    , TcPluginProgress -> [Ct]
pluginNewCts    :: [Ct]
      -- ^ New constraints emitted by plugins
    }

getTcPluginSolvers :: TcS [TcPluginSolver]
getTcPluginSolvers :: TcS [TcPluginSolver]
getTcPluginSolvers
  = do { TcGblEnv
tcg_env <- TcS TcGblEnv
getGblEnv; [TcPluginSolver] -> TcS [TcPluginSolver]
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (TcGblEnv -> [TcPluginSolver]
tcg_tc_plugin_solvers TcGblEnv
tcg_env) }

-- | Starting from a pair of (given, wanted) constraints,
-- invoke each of the typechecker constraint-solving plugins in turn and return
--
--  * the remaining unmodified constraints,
--  * constraints that have been solved,
--  * constraints that are insoluble, and
--  * new work.
--
-- Note that new work generated by one plugin will not be seen by
-- other plugins on this pass (but the main constraint solver will be
-- re-invoked and they will see it later).  There is no check that new
-- work differs from the original constraints supplied to the plugin:
-- the plugin itself should perform this check if necessary.
runTcPluginSolvers :: [TcPluginSolver] -> SplitCts -> TcS TcPluginProgress
runTcPluginSolvers :: [TcPluginSolver] -> SplitCts -> TcS TcPluginProgress
runTcPluginSolvers [TcPluginSolver]
solvers SplitCts
all_cts
  = do { EvBindsVar
ev_binds_var <- TcS EvBindsVar
getTcEvBindsVar
       ; (TcPluginProgress -> TcPluginSolver -> TcS TcPluginProgress)
-> TcPluginProgress -> [TcPluginSolver] -> TcS TcPluginProgress
forall (t :: * -> *) (m :: * -> *) b a.
(Foldable t, Monad m) =>
(b -> a -> m b) -> b -> t a -> m b
foldM (EvBindsVar
-> TcPluginProgress -> TcPluginSolver -> TcS TcPluginProgress
do_plugin EvBindsVar
ev_binds_var) TcPluginProgress
initialProgress [TcPluginSolver]
solvers }
  where
    do_plugin :: EvBindsVar -> TcPluginProgress -> TcPluginSolver -> TcS TcPluginProgress
    do_plugin :: EvBindsVar
-> TcPluginProgress -> TcPluginSolver -> TcS TcPluginProgress
do_plugin EvBindsVar
ev_binds_var TcPluginProgress
p TcPluginSolver
solver = do
        TcPluginSolveResult
result <- TcPluginM TcPluginSolveResult -> TcS TcPluginSolveResult
forall a. TcPluginM a -> TcS a
runTcPluginTcS (([Ct] -> [Ct] -> TcPluginM TcPluginSolveResult)
-> SplitCts -> TcPluginM TcPluginSolveResult
forall a b c. (a -> b -> c) -> (a, b) -> c
uncurry (TcPluginSolver
solver EvBindsVar
ev_binds_var) (TcPluginProgress -> SplitCts
pluginInputCts TcPluginProgress
p))
        TcPluginProgress -> TcS TcPluginProgress
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (TcPluginProgress -> TcS TcPluginProgress)
-> TcPluginProgress -> TcS TcPluginProgress
forall a b. (a -> b) -> a -> b
$ TcPluginProgress -> TcPluginSolveResult -> TcPluginProgress
progress TcPluginProgress
p TcPluginSolveResult
result

    progress :: TcPluginProgress -> TcPluginSolveResult -> TcPluginProgress
    progress :: TcPluginProgress -> TcPluginSolveResult -> TcPluginProgress
progress TcPluginProgress
p
      (TcPluginSolveResult
        { tcPluginInsolubleCts :: TcPluginSolveResult -> [Ct]
tcPluginInsolubleCts = [Ct]
bad_cts
        , tcPluginSolvedCts :: TcPluginSolveResult -> [(EvTerm, Ct)]
tcPluginSolvedCts    = [(EvTerm, Ct)]
solved_cts
        , tcPluginNewCts :: TcPluginSolveResult -> [Ct]
tcPluginNewCts       = [Ct]
new_cts
        }
      ) =
        TcPluginProgress
p { pluginInputCts  = discard (bad_cts ++ map snd solved_cts) (pluginInputCts p)
          , pluginSolvedCts = add solved_cts (pluginSolvedCts p)
          , pluginNewCts    = new_cts ++ pluginNewCts p
          , pluginBadCts    = bad_cts ++ pluginBadCts p
          }

    initialProgress :: TcPluginProgress
initialProgress = SplitCts
-> ([Ct], [(EvTerm, Ct)]) -> [Ct] -> [Ct] -> TcPluginProgress
TcPluginProgress SplitCts
all_cts ([], []) [] []

    discard :: [Ct] -> SplitCts -> SplitCts
    discard :: [Ct] -> SplitCts -> SplitCts
discard [Ct]
cts ([Ct]
xs, [Ct]
ys) =
        ([Ct]
xs [Ct] -> [Ct] -> [Ct]
`without` [Ct]
cts, [Ct]
ys [Ct] -> [Ct] -> [Ct]
`without` [Ct]
cts)

    without :: [Ct] -> [Ct] -> [Ct]
    without :: [Ct] -> [Ct] -> [Ct]
without = (Ct -> Ct -> Bool) -> [Ct] -> [Ct] -> [Ct]
forall a. (a -> a -> Bool) -> [a] -> [a] -> [a]
deleteFirstsBy Ct -> Ct -> Bool
eqCt

    eqCt :: Ct -> Ct -> Bool
    eqCt :: Ct -> Ct -> Bool
eqCt Ct
c Ct
c' = Ct -> CtFlavour
ctFlavour Ct
c CtFlavour -> CtFlavour -> Bool
forall a. Eq a => a -> a -> Bool
== Ct -> CtFlavour
ctFlavour Ct
c'
             Bool -> Bool -> Bool
&& Ct -> TcPredType
ctPred Ct
c (() :: Constraint) => TcPredType -> TcPredType -> Bool
TcPredType -> TcPredType -> Bool
`tcEqType` Ct -> TcPredType
ctPred Ct
c'

    add :: [(EvTerm,Ct)] -> SolvedCts -> SolvedCts
    add :: [(EvTerm, Ct)] -> ([Ct], [(EvTerm, Ct)]) -> ([Ct], [(EvTerm, Ct)])
add [(EvTerm, Ct)]
xs ([Ct], [(EvTerm, Ct)])
scs = (([Ct], [(EvTerm, Ct)]) -> (EvTerm, Ct) -> ([Ct], [(EvTerm, Ct)]))
-> ([Ct], [(EvTerm, Ct)])
-> [(EvTerm, Ct)]
-> ([Ct], [(EvTerm, Ct)])
forall b a. (b -> a -> b) -> b -> [a] -> b
forall (t :: * -> *) b a.
Foldable t =>
(b -> a -> b) -> b -> t a -> b
foldl' ([Ct], [(EvTerm, Ct)]) -> (EvTerm, Ct) -> ([Ct], [(EvTerm, Ct)])
addOne ([Ct], [(EvTerm, Ct)])
scs [(EvTerm, Ct)]
xs

    addOne :: SolvedCts -> (EvTerm,Ct) -> SolvedCts
    addOne :: ([Ct], [(EvTerm, Ct)]) -> (EvTerm, Ct) -> ([Ct], [(EvTerm, Ct)])
addOne ([Ct]
givens, [(EvTerm, Ct)]
wanteds) (EvTerm
ev,Ct
ct) = case Ct -> CtEvidence
ctEvidence Ct
ct of
      CtGiven  {} -> (Ct
ctCt -> [Ct] -> [Ct]
forall a. a -> [a] -> [a]
:[Ct]
givens, [(EvTerm, Ct)]
wanteds)
      CtWanted {} -> ([Ct]
givens, (EvTerm
ev,Ct
ct)(EvTerm, Ct) -> [(EvTerm, Ct)] -> [(EvTerm, Ct)]
forall a. a -> [a] -> [a]
:[(EvTerm, Ct)]
wanteds)


type WorkItem = Ct
type SimplifierStage = WorkItem -> TcS (StopOrContinue Ct)

runSolverPipeline :: [(String,SimplifierStage)] -- The pipeline
                  -> WorkItem                   -- The work item
                  -> TcS ()
-- Run this item down the pipeline, leaving behind new work and inerts
runSolverPipeline :: [(String, SimplifierStage)] -> Ct -> TcS ()
runSolverPipeline [(String, SimplifierStage)]
pipeline Ct
workItem
  = do { WorkList
wl <- TcS WorkList
getWorkList
       ; InertSet
inerts <- TcS InertSet
getTcSInerts
       ; TcLevel
tclevel <- TcS TcLevel
getTcLevel
       ; String -> SDoc -> TcS ()
traceTcS String
"----------------------------- " SDoc
forall doc. IsOutput doc => doc
empty
       ; String -> SDoc -> TcS ()
traceTcS String
"Start solver pipeline {" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
                  [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"tclevel =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcLevel -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcLevel
tclevel
                       , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"work item =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
workItem
                       , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"inerts =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> InertSet -> SDoc
forall a. Outputable a => a -> SDoc
ppr InertSet
inerts
                       , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"rest of worklist =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> WorkList -> SDoc
forall a. Outputable a => a -> SDoc
ppr WorkList
wl ]

       ; TcS ()
bumpStepCountTcS    -- One step for each constraint processed
       ; StopOrContinue Ct
final_res  <- [(String, SimplifierStage)]
-> StopOrContinue Ct -> TcS (StopOrContinue Ct)
run_pipeline [(String, SimplifierStage)]
pipeline (Ct -> StopOrContinue Ct
forall a. a -> StopOrContinue a
ContinueWith Ct
workItem)

       ; case StopOrContinue Ct
final_res of
           Stop CtEvidence
ev SDoc
s       -> do { CtEvidence -> SDoc -> TcS ()
traceFireTcS CtEvidence
ev SDoc
s
                                 ; String -> SDoc -> TcS ()
traceTcS String
"End solver pipeline (discharged) }" SDoc
forall doc. IsOutput doc => doc
empty
                                 ; () -> TcS ()
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return () }
           ContinueWith Ct
ct -> do { Ct -> TcS ()
addInertCan Ct
ct
                                 ; CtEvidence -> SDoc -> TcS ()
traceFireTcS (Ct -> CtEvidence
ctEvidence Ct
ct) (String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Kept as inert")
                                 ; String -> SDoc -> TcS ()
traceTcS String
"End solver pipeline (kept as inert) }" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
                                            (String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"final_item =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
ct) }
       }
  where run_pipeline :: [(String,SimplifierStage)] -> StopOrContinue Ct
                     -> TcS (StopOrContinue Ct)
        run_pipeline :: [(String, SimplifierStage)]
-> StopOrContinue Ct -> TcS (StopOrContinue Ct)
run_pipeline [] StopOrContinue Ct
res        = StopOrContinue Ct -> TcS (StopOrContinue Ct)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return StopOrContinue Ct
res
        run_pipeline [(String, SimplifierStage)]
_ (Stop CtEvidence
ev SDoc
s) = StopOrContinue Ct -> TcS (StopOrContinue Ct)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (CtEvidence -> SDoc -> StopOrContinue Ct
forall a. CtEvidence -> SDoc -> StopOrContinue a
Stop CtEvidence
ev SDoc
s)
        run_pipeline ((String
stg_name,SimplifierStage
stg):[(String, SimplifierStage)]
stgs) (ContinueWith Ct
ct)
          = do { String -> SDoc -> TcS ()
traceTcS (String
"runStage " String -> String -> String
forall a. [a] -> [a] -> [a]
++ String
stg_name String -> String -> String
forall a. [a] -> [a] -> [a]
++ String
" {")
                          (String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"workitem   = " SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
ct)
               ; StopOrContinue Ct
res <- SimplifierStage
stg Ct
ct
               ; String -> SDoc -> TcS ()
traceTcS (String
"end stage " String -> String -> String
forall a. [a] -> [a] -> [a]
++ String
stg_name String -> String -> String
forall a. [a] -> [a] -> [a]
++ String
" }") SDoc
forall doc. IsOutput doc => doc
empty
               ; [(String, SimplifierStage)]
-> StopOrContinue Ct -> TcS (StopOrContinue Ct)
run_pipeline [(String, SimplifierStage)]
stgs StopOrContinue Ct
res }

{-
Example 1:
  Inert:   {c ~ d, F a ~ t, b ~ Int, a ~ ty} (all given)
  Reagent: a ~ [b] (given)

React with (c~d)     ==> IR (ContinueWith (a~[b]))  True    []
React with (F a ~ t) ==> IR (ContinueWith (a~[b]))  False   [F [b] ~ t]
React with (b ~ Int) ==> IR (ContinueWith (a~[Int]) True    []

Example 2:
  Inert:  {c ~w d, F a ~g t, b ~w Int, a ~w ty}
  Reagent: a ~w [b]

React with (c ~w d)   ==> IR (ContinueWith (a~[b]))  True    []
React with (F a ~g t) ==> IR (ContinueWith (a~[b]))  True    []    (can't rewrite given with wanted!)
etc.

Example 3:
  Inert:  {a ~ Int, F Int ~ b} (given)
  Reagent: F a ~ b (wanted)

React with (a ~ Int)   ==> IR (ContinueWith (F Int ~ b)) True []
React with (F Int ~ b) ==> IR Stop True []    -- after substituting we re-canonicalize and get nothing
-}

thePipeline :: [(String,SimplifierStage)]
thePipeline :: [(String, SimplifierStage)]
thePipeline = [ (String
"canonicalization",        SimplifierStage
GHC.Tc.Solver.Canonical.canonicalize)
              , (String
"interact with inerts",    SimplifierStage
interactWithInertsStage)
              , (String
"top-level reactions",     SimplifierStage
topReactionsStage) ]

{-
*********************************************************************************
*                                                                               *
                       The interact-with-inert Stage
*                                                                               *
*********************************************************************************

Note [The Solver Invariant]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
We always add Givens first.  So you might think that the solver has
the invariant

   If the work-item is Given,
   then the inert item must Given

But this isn't quite true.  Suppose we have,
    c1: [W] beta ~ [alpha], c2 : [W] blah, c3 :[W] alpha ~ Int
After processing the first two, we get
     c1: [G] beta ~ [alpha], c2 : [W] blah
Now, c3 does not interact with the given c1, so when we spontaneously
solve c3, we must re-react it with the inert set.  So we can attempt a
reaction between inert c2 [W] and work-item c3 [G].

It *is* true that [Solver Invariant]
   If the work-item is Given,
   AND there is a reaction
   then the inert item must Given
or, equivalently,
   If the work-item is Given,
   and the inert item is Wanted
   then there is no reaction
-}

-- Interaction result of  WorkItem <~> Ct

interactWithInertsStage :: WorkItem -> TcS (StopOrContinue Ct)
-- Precondition: if the workitem is a CEqCan then it will not be able to
-- react with anything at this stage (except, maybe, via a type family
-- dependency)

interactWithInertsStage :: SimplifierStage
interactWithInertsStage Ct
wi
  = do { InertSet
inerts <- TcS InertSet
getTcSInerts
       ; let ics :: InertCans
ics = InertSet -> InertCans
inert_cans InertSet
inerts
       ; case Ct
wi of
             CEqCan       {} -> InertCans -> SimplifierStage
interactEq      InertCans
ics Ct
wi
             CIrredCan    {} -> InertCans -> SimplifierStage
interactIrred   InertCans
ics Ct
wi
             CDictCan     {} -> InertCans -> SimplifierStage
interactDict    InertCans
ics Ct
wi
             Ct
_ -> String -> SDoc -> TcS (StopOrContinue Ct)
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"interactWithInerts" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
wi) }
                -- CNonCanonical have been canonicalised

data InteractResult
   = KeepInert   -- Keep the inert item, and solve the work item from it
                 -- (if the latter is Wanted; just discard it if not)
   | KeepWork    -- Keep the work item, and solve the inert item from it

instance Outputable InteractResult where
  ppr :: InteractResult -> SDoc
ppr InteractResult
KeepInert = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"keep inert"
  ppr InteractResult
KeepWork  = String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"keep work-item"

solveOneFromTheOther :: Ct  -- Inert    (Dict or Irred)
                     -> Ct  -- WorkItem (same predicate as inert)
                     -> InteractResult
-- Precondition:
-- * inert and work item represent evidence for the /same/ predicate
-- * Both are CDictCan or CIrredCan
--
-- We can always solve one from the other: even if both are wanted,
-- although we don't rewrite wanteds with wanteds, we can combine
-- two wanteds into one by solving one from the other

solveOneFromTheOther :: Ct -> Ct -> InteractResult
solveOneFromTheOther Ct
ct_i Ct
ct_w
  | CtWanted { ctev_loc :: CtEvidence -> CtLoc
ctev_loc = CtLoc
loc_w } <- CtEvidence
ev_w
  , CtLoc -> CtLoc -> Bool
prohibitedSuperClassSolve CtLoc
loc_i CtLoc
loc_w
  -- See Note [Solving superclass constraints] in GHC.Tc.TyCl.Instance
  = -- Inert must be Given
    InteractResult
KeepWork

  | CtWanted {} <- CtEvidence
ev_w
  = -- Inert is Given or Wanted
    case CtEvidence
ev_i of
      CtGiven {} -> InteractResult
KeepInert
        -- work is Wanted; inert is Given: easy choice.

      CtWanted {} -- Both are Wanted
        -- If only one has no pending superclasses, use it
        -- Otherwise we can get infinite superclass expansion (#22516)
        -- in silly cases like   class C T b => C a b where ...
        | Bool -> Bool
not Bool
is_psc_i, Bool
is_psc_w     -> InteractResult
KeepInert
        | Bool
is_psc_i,     Bool -> Bool
not Bool
is_psc_w -> InteractResult
KeepWork

        -- If only one is a WantedSuperclassOrigin (arising from expanding
        -- a Wanted class constraint), keep the other: wanted superclasses
        -- may be unexpected by users
        | Bool -> Bool
not Bool
is_wsc_orig_i, Bool
is_wsc_orig_w     -> InteractResult
KeepInert
        | Bool
is_wsc_orig_i,     Bool -> Bool
not Bool
is_wsc_orig_w -> InteractResult
KeepWork

        -- otherwise, just choose the lower span
        -- reason: if we have something like (abs 1) (where the
        -- Num constraint cannot be satisfied), it's better to
        -- get an error about abs than about 1.
        -- This test might become more elaborate if we see an
        -- opportunity to improve the error messages
        | (RealSrcSpan -> RealSrcSpan -> Bool
forall a. Ord a => a -> a -> Bool
(<) (RealSrcSpan -> RealSrcSpan -> Bool)
-> (CtLoc -> RealSrcSpan) -> CtLoc -> CtLoc -> Bool
forall b c a. (b -> b -> c) -> (a -> b) -> a -> a -> c
`on` CtLoc -> RealSrcSpan
ctLocSpan) CtLoc
loc_i CtLoc
loc_w -> InteractResult
KeepInert
        | Bool
otherwise                        -> InteractResult
KeepWork

  -- From here on the work-item is Given

  | CtWanted { ctev_loc :: CtEvidence -> CtLoc
ctev_loc = CtLoc
loc_i } <- CtEvidence
ev_i
  , CtLoc -> CtLoc -> Bool
prohibitedSuperClassSolve CtLoc
loc_w CtLoc
loc_i
  = InteractResult
KeepInert   -- Just discard the un-usable Given
                -- This never actually happens because
                -- Givens get processed first

  | CtWanted {} <- CtEvidence
ev_i
  = InteractResult
KeepWork

  -- From here on both are Given
  -- See Note [Replacement vs keeping]

  | TcLevel
lvl_i TcLevel -> TcLevel -> Bool
forall a. Eq a => a -> a -> Bool
== TcLevel
lvl_w
  = InteractResult
same_level_strategy

  | Bool
otherwise   -- Both are Given, levels differ
  = InteractResult
different_level_strategy
  where
     ev_i :: CtEvidence
ev_i  = Ct -> CtEvidence
ctEvidence Ct
ct_i
     ev_w :: CtEvidence
ev_w  = Ct -> CtEvidence
ctEvidence Ct
ct_w

     pred :: TcPredType
pred  = CtEvidence -> TcPredType
ctEvPred CtEvidence
ev_i

     loc_i :: CtLoc
loc_i  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev_i
     loc_w :: CtLoc
loc_w  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev_w
     orig_i :: CtOrigin
orig_i = CtLoc -> CtOrigin
ctLocOrigin CtLoc
loc_i
     orig_w :: CtOrigin
orig_w = CtLoc -> CtOrigin
ctLocOrigin CtLoc
loc_w
     lvl_i :: TcLevel
lvl_i  = CtLoc -> TcLevel
ctLocLevel CtLoc
loc_i
     lvl_w :: TcLevel
lvl_w  = CtLoc -> TcLevel
ctLocLevel CtLoc
loc_w

     is_psc_w :: Bool
is_psc_w = Ct -> Bool
isPendingScDict Ct
ct_w
     is_psc_i :: Bool
is_psc_i = Ct -> Bool
isPendingScDict Ct
ct_i

     is_wsc_orig_i :: Bool
is_wsc_orig_i = CtOrigin -> Bool
isWantedSuperclassOrigin CtOrigin
orig_i
     is_wsc_orig_w :: Bool
is_wsc_orig_w = CtOrigin -> Bool
isWantedSuperclassOrigin CtOrigin
orig_w

     different_level_strategy :: InteractResult
different_level_strategy  -- Both Given
       | TcPredType -> Bool
isIPLikePred TcPredType
pred = if TcLevel
lvl_w TcLevel -> TcLevel -> Bool
forall a. Ord a => a -> a -> Bool
> TcLevel
lvl_i then InteractResult
KeepWork  else InteractResult
KeepInert
       | Bool
otherwise         = if TcLevel
lvl_w TcLevel -> TcLevel -> Bool
forall a. Ord a => a -> a -> Bool
> TcLevel
lvl_i then InteractResult
KeepInert else InteractResult
KeepWork
       -- See Note [Replacement vs keeping] part (1)
       -- For the isIPLikePred case see Note [Shadowing of Implicit Parameters]

     same_level_strategy :: InteractResult
same_level_strategy -- Both Given
       = case (CtOrigin
orig_i, CtOrigin
orig_w) of

           (GivenSCOrigin SkolemInfoAnon
_ ScDepth
depth_i Bool
blocked_i, GivenSCOrigin SkolemInfoAnon
_ ScDepth
depth_w Bool
blocked_w)
             | Bool
blocked_i, Bool -> Bool
not Bool
blocked_w -> InteractResult
KeepWork  -- Case 2(a) from
             | Bool -> Bool
not Bool
blocked_i, Bool
blocked_w -> InteractResult
KeepInert -- Note [Replacement vs keeping]

             -- Both blocked or both not blocked

             | ScDepth
depth_w ScDepth -> ScDepth -> Bool
forall a. Ord a => a -> a -> Bool
< ScDepth
depth_i -> InteractResult
KeepWork   -- Case 2(c) from
             | Bool
otherwise         -> InteractResult
KeepInert  -- Note [Replacement vs keeping]

           (GivenSCOrigin {}, CtOrigin
_) -> InteractResult
KeepWork  -- Case 2(b) from Note [Replacement vs keeping]

           (CtOrigin, CtOrigin)
_ -> InteractResult
KeepInert  -- Case 2(d) from Note [Replacement vs keeping]

{-
Note [Replacement vs keeping]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we have two Given constraints both of type (C tys), say, which should
we keep?  More subtle than you might think! This is all implemented in
solveOneFromTheOther.

  1) Constraints come from different levels (different_level_strategy)

      - For implicit parameters we want to keep the innermost (deepest)
        one, so that it overrides the outer one.
        See Note [Shadowing of Implicit Parameters]

      - For everything else, we want to keep the outermost one.  Reason: that
        makes it more likely that the inner one will turn out to be unused,
        and can be reported as redundant.  See Note [Tracking redundant constraints]
        in GHC.Tc.Solver.

        It transpires that using the outermost one is responsible for an
        8% performance improvement in nofib cryptarithm2, compared to
        just rolling the dice.  I didn't investigate why.

  2) Constraints coming from the same level (i.e. same implication)

       (a) If both are GivenSCOrigin, choose the one that is unblocked if possible
           according to Note [Solving superclass constraints] in GHC.Tc.TyCl.Instance.

       (b) Prefer constraints that are not superclass selections. Example:

             f :: (Eq a, Ord a) => a -> Bool
             f x = x == x

           Eager superclass expansion gives us two [G] Eq a constraints. We
           want to keep the one from the user-written Eq a, not the superclass
           selection. This means we report the Ord a as redundant with
           -Wredundant-constraints, not the Eq a.

           Getting this wrong was #20602. See also
           Note [Tracking redundant constraints] in GHC.Tc.Solver.

       (c) If both are GivenSCOrigin, chooose the one with the shallower
           superclass-selection depth, in the hope of identifying more correct
           redundant constraints. This is really a generalization of point (b),
           because the superclass depth of a non-superclass constraint is 0.

           (If the levels differ, we definitely won't have both with GivenSCOrigin.)

       (d) Finally, when there is still a choice, use KeepInert rather than
           KeepWork, for two reasons:
             - to avoid unnecessary munging of the inert set.
             - to cut off superclass loops; see Note [Superclass loops] in GHC.Tc.Solver.Canonical

Doing the level-check for implicit parameters, rather than making the work item
always override, is important.  Consider

    data T a where { T1 :: (?x::Int) => T Int; T2 :: T a }

    f :: (?x::a) => T a -> Int
    f T1 = ?x
    f T2 = 3

We have a [G] (?x::a) in the inert set, and at the pattern match on T1 we add
two new givens in the work-list:  [G] (?x::Int)
                                  [G] (a ~ Int)
Now consider these steps
  - process a~Int, kicking out (?x::a)
  - process (?x::Int), the inner given, adding to inert set
  - process (?x::a), the outer given, overriding the inner given
Wrong!  The level-check ensures that the inner implicit parameter wins.
(Actually I think that the order in which the work-list is processed means
that this chain of events won't happen, but that's very fragile.)

*********************************************************************************
*                                                                               *
                   interactIrred
*                                                                               *
*********************************************************************************

Note [Multiple matching irreds]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
You might think that it's impossible to have multiple irreds all match the
work item; after all, interactIrred looks for matches and solves one from the
other. However, note that interacting insoluble, non-droppable irreds does not
do this matching. We thus might end up with several insoluble, non-droppable,
matching irreds in the inert set. When another irred comes along that we have
not yet labeled insoluble, we can find multiple matches. These multiple matches
cause no harm, but it would be wrong to ASSERT that they aren't there (as we
once had done). This problem can be tickled by typecheck/should_compile/holes.

-}

-- Two pieces of irreducible evidence: if their types are *exactly identical*
-- we can rewrite them. We can never improve using this:
-- if we want ty1 :: Constraint and have ty2 :: Constraint it clearly does not
-- mean that (ty1 ~ ty2)
interactIrred :: InertCans -> Ct -> TcS (StopOrContinue Ct)

interactIrred :: InertCans -> SimplifierStage
interactIrred InertCans
inerts ct_w :: Ct
ct_w@(CIrredCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev_w, cc_reason :: Ct -> CtIrredReason
cc_reason = CtIrredReason
reason })
  | CtIrredReason -> Bool
isInsolubleReason CtIrredReason
reason
               -- For insolubles, don't allow the constraint to be dropped
               -- which can happen with solveOneFromTheOther, so that
               -- we get distinct error messages with -fdefer-type-errors
  = SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
ct_w

  | let (Bag (Ct, SwapFlag)
matching_irreds, Cts
others) = Cts -> CtEvidence -> (Bag (Ct, SwapFlag), Cts)
findMatchingIrreds (InertCans -> Cts
inert_irreds InertCans
inerts) CtEvidence
ev_w
  , ((Ct
ct_i, SwapFlag
swap) : [(Ct, SwapFlag)]
_rest) <- Bag (Ct, SwapFlag) -> [(Ct, SwapFlag)]
forall a. Bag a -> [a]
bagToList Bag (Ct, SwapFlag)
matching_irreds
        -- See Note [Multiple matching irreds]
  , let ev_i :: CtEvidence
ev_i = Ct -> CtEvidence
ctEvidence Ct
ct_i
  = do { String -> SDoc -> TcS ()
traceTcS String
"iteractIrred" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
         [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"wanted:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
ct_w SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ CtOrigin -> SDoc
forall a. Outputable a => a -> SDoc
ppr (Ct -> CtOrigin
ctOrigin Ct
ct_w))
              , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"inert: " SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
ct_i SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ CtOrigin -> SDoc
forall a. Outputable a => a -> SDoc
ppr (Ct -> CtOrigin
ctOrigin Ct
ct_i)) ]
       ; case Ct -> Ct -> InteractResult
solveOneFromTheOther Ct
ct_i Ct
ct_w of
            InteractResult
KeepInert -> do { CtEvidence -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev_w (SwapFlag -> CtEvidence -> EvTerm
swap_me SwapFlag
swap CtEvidence
ev_i)
                            ; StopOrContinue Ct -> TcS (StopOrContinue Ct)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (CtEvidence -> SDoc -> StopOrContinue Ct
forall a. CtEvidence -> SDoc -> StopOrContinue a
Stop CtEvidence
ev_w (String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Irred equal:KeepInert" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
ct_w)) }
            InteractResult
KeepWork ->  do { CtEvidence -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev_i (SwapFlag -> CtEvidence -> EvTerm
swap_me SwapFlag
swap CtEvidence
ev_w)
                            ; (Cts -> Cts) -> TcS ()
updInertIrreds (\Cts
_ -> Cts
others)
                            ; SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
ct_w } }

  | Bool
otherwise
  = SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
ct_w

  where
    swap_me :: SwapFlag -> CtEvidence -> EvTerm
    swap_me :: SwapFlag -> CtEvidence -> EvTerm
swap_me SwapFlag
swap CtEvidence
ev
      = case SwapFlag
swap of
           SwapFlag
NotSwapped -> CtEvidence -> EvTerm
ctEvTerm CtEvidence
ev
           SwapFlag
IsSwapped  -> TcCoercion -> EvTerm
evCoercion (TcCoercion -> TcCoercion
mkSymCo (EvTerm -> TcCoercion
evTermCoercion (CtEvidence -> EvTerm
ctEvTerm CtEvidence
ev)))

interactIrred InertCans
_ Ct
wi = String -> SDoc -> TcS (StopOrContinue Ct)
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"interactIrred" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
wi)

findMatchingIrreds :: Cts -> CtEvidence -> (Bag (Ct, SwapFlag), Bag Ct)
findMatchingIrreds :: Cts -> CtEvidence -> (Bag (Ct, SwapFlag), Cts)
findMatchingIrreds Cts
irreds CtEvidence
ev
  | EqPred EqRel
eq_rel1 TcPredType
lty1 TcPredType
rty1 <- TcPredType -> Pred
classifyPredType TcPredType
pred
    -- See Note [Solving irreducible equalities]
  = (Ct -> Either (Ct, SwapFlag) Ct)
-> Cts -> (Bag (Ct, SwapFlag), Cts)
forall a b c. (a -> Either b c) -> Bag a -> (Bag b, Bag c)
partitionBagWith (EqRel -> TcPredType -> TcPredType -> Ct -> Either (Ct, SwapFlag) Ct
match_eq EqRel
eq_rel1 TcPredType
lty1 TcPredType
rty1) Cts
irreds
  | Bool
otherwise
  = (Ct -> Either (Ct, SwapFlag) Ct)
-> Cts -> (Bag (Ct, SwapFlag), Cts)
forall a b c. (a -> Either b c) -> Bag a -> (Bag b, Bag c)
partitionBagWith Ct -> Either (Ct, SwapFlag) Ct
match_non_eq Cts
irreds
  where
    pred :: TcPredType
pred = CtEvidence -> TcPredType
ctEvPred CtEvidence
ev
    match_non_eq :: Ct -> Either (Ct, SwapFlag) Ct
match_non_eq Ct
ct
      | Ct -> TcPredType
ctPred Ct
ct TcPredType -> TcPredType -> Bool
`tcEqTypeNoKindCheck` TcPredType
pred = (Ct, SwapFlag) -> Either (Ct, SwapFlag) Ct
forall a b. a -> Either a b
Left (Ct
ct, SwapFlag
NotSwapped)
      | Bool
otherwise                            = Ct -> Either (Ct, SwapFlag) Ct
forall a b. b -> Either a b
Right Ct
ct

    match_eq :: EqRel -> TcPredType -> TcPredType -> Ct -> Either (Ct, SwapFlag) Ct
match_eq EqRel
eq_rel1 TcPredType
lty1 TcPredType
rty1 Ct
ct
      | EqPred EqRel
eq_rel2 TcPredType
lty2 TcPredType
rty2 <- TcPredType -> Pred
classifyPredType (Ct -> TcPredType
ctPred Ct
ct)
      , EqRel
eq_rel1 EqRel -> EqRel -> Bool
forall a. Eq a => a -> a -> Bool
== EqRel
eq_rel2
      , Just SwapFlag
swap <- TcPredType
-> TcPredType -> TcPredType -> TcPredType -> Maybe SwapFlag
match_eq_help TcPredType
lty1 TcPredType
rty1 TcPredType
lty2 TcPredType
rty2
      = (Ct, SwapFlag) -> Either (Ct, SwapFlag) Ct
forall a b. a -> Either a b
Left (Ct
ct, SwapFlag
swap)
      | Bool
otherwise
      = Ct -> Either (Ct, SwapFlag) Ct
forall a b. b -> Either a b
Right Ct
ct

    match_eq_help :: TcPredType
-> TcPredType -> TcPredType -> TcPredType -> Maybe SwapFlag
match_eq_help TcPredType
lty1 TcPredType
rty1 TcPredType
lty2 TcPredType
rty2
      | TcPredType
lty1 TcPredType -> TcPredType -> Bool
`tcEqTypeNoKindCheck` TcPredType
lty2, TcPredType
rty1 TcPredType -> TcPredType -> Bool
`tcEqTypeNoKindCheck` TcPredType
rty2
      = SwapFlag -> Maybe SwapFlag
forall a. a -> Maybe a
Just SwapFlag
NotSwapped
      | TcPredType
lty1 TcPredType -> TcPredType -> Bool
`tcEqTypeNoKindCheck` TcPredType
rty2, TcPredType
rty1 TcPredType -> TcPredType -> Bool
`tcEqTypeNoKindCheck` TcPredType
lty2
      = SwapFlag -> Maybe SwapFlag
forall a. a -> Maybe a
Just SwapFlag
IsSwapped
      | Bool
otherwise
      = Maybe SwapFlag
forall a. Maybe a
Nothing

{- Note [Solving irreducible equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider (#14333)
  [G] a b ~R# c d
  [W] c d ~R# a b
Clearly we should be able to solve this! Even though the constraints are
not decomposable. We solve this when looking up the work-item in the
irreducible constraints to look for an identical one.  When doing this
lookup, findMatchingIrreds spots the equality case, and matches either
way around. It has to return a swap-flag so we can generate evidence
that is the right way round too.
-}

{-
*********************************************************************************
*                                                                               *
                   interactDict
*                                                                               *
*********************************************************************************

Note [Shortcut solving]
~~~~~~~~~~~~~~~~~~~~~~~
When we interact a [W] constraint with a [G] constraint that solves it, there is
a possibility that we could produce better code if instead we solved from a
top-level instance declaration (See #12791, #5835). For example:

    class M a b where m :: a -> b

    type C a b = (Num a, M a b)

    f :: C Int b => b -> Int -> Int
    f _ x = x + 1

The body of `f` requires a [W] `Num Int` instance. We could solve this
constraint from the givens because we have `C Int b` and that provides us a
solution for `Num Int`. This would let us produce core like the following
(with -O2):

    f :: forall b. C Int b => b -> Int -> Int
    f = \ (@ b) ($d(%,%) :: C Int b) _ (eta1 :: Int) ->
        + @ Int
          (GHC.Classes.$p1(%,%) @ (Num Int) @ (M Int b) $d(%,%))
          eta1
          A.f1

This is bad! We could do /much/ better if we solved [W] `Num Int` directly
from the instance that we have in scope:

    f :: forall b. C Int b => b -> Int -> Int
    f = \ (@ b) _ _ (x :: Int) ->
        case x of { GHC.Types.I# x1 -> GHC.Types.I# (GHC.Prim.+# x1 1#) }

** NB: It is important to emphasize that all this is purely an optimization:
** exactly the same programs should typecheck with or without this
** procedure.

Solving fully
~~~~~~~~~~~~~
There is a reason why the solver does not simply try to solve such
constraints with top-level instances. If the solver finds a relevant
instance declaration in scope, that instance may require a context
that can't be solved for. A good example of this is:

    f :: Ord [a] => ...
    f x = ..Need Eq [a]...

If we have instance `Eq a => Eq [a]` in scope and we tried to use it, we would
be left with the obligation to solve the constraint Eq a, which we cannot. So we
must be conservative in our attempt to use an instance declaration to solve the
[W] constraint we're interested in.

Our rule is that we try to solve all of the instance's subgoals
recursively all at once. Precisely: We only attempt to solve
constraints of the form `C1, ... Cm => C t1 ... t n`, where all the Ci
are themselves class constraints of the form `C1', ... Cm' => C' t1'
... tn'` and we only succeed if the entire tree of constraints is
solvable from instances.

An example that succeeds:

    class Eq a => C a b | b -> a where
      m :: b -> a

    f :: C [Int] b => b -> Bool
    f x = m x == []

We solve for `Eq [Int]`, which requires `Eq Int`, which we also have. This
produces the following core:

    f :: forall b. C [Int] b => b -> Bool
    f = \ (@ b) ($dC :: C [Int] b) (x :: b) ->
        GHC.Classes.$fEq[]_$s$c==
          (m @ [Int] @ b $dC x) (GHC.Types.[] @ Int)

An example that fails:

    class Eq a => C a b | b -> a where
      m :: b -> a

    f :: C [a] b => b -> Bool
    f x = m x == []

Which, because solving `Eq [a]` demands `Eq a` which we cannot solve, produces:

    f :: forall a b. C [a] b => b -> Bool
    f = \ (@ a) (@ b) ($dC :: C [a] b) (eta :: b) ->
        ==
          @ [a]
          (A.$p1C @ [a] @ b $dC)
          (m @ [a] @ b $dC eta)
          (GHC.Types.[] @ a)

Note [Shortcut solving: type families]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have (#13943)
  class Take (n :: Nat) where ...
  instance {-# OVERLAPPING #-}                    Take 0 where ..
  instance {-# OVERLAPPABLE #-} (Take (n - 1)) => Take n where ..

And we have [W] Take 3.  That only matches one instance so we get
[W] Take (3-1).  Really we should now rewrite to reduce the (3-1) to 2, and
so on -- but that is reproducing yet more of the solver.  Sigh.  For now,
we just give up (remember all this is just an optimisation).

But we must not just naively try to lookup (Take (3-1)) in the
InstEnv, or it'll (wrongly) appear not to match (Take 0) and get a
unique match on the (Take n) instance.  That leads immediately to an
infinite loop.  Hence the check that 'preds' have no type families
(isTyFamFree).

Note [Shortcut solving: incoherence]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This optimization relies on coherence of dictionaries to be correct. When we
cannot assume coherence because of IncoherentInstances then this optimization
can change the behavior of the user's code.

The following four modules produce a program whose output would change depending
on whether we apply this optimization when IncoherentInstances is in effect:

=========
    {-# LANGUAGE MultiParamTypeClasses #-}
    module A where

    class A a where
      int :: a -> Int

    class A a => C a b where
      m :: b -> a -> a

=========
    {-# LANGUAGE FlexibleInstances     #-}
    {-# LANGUAGE MultiParamTypeClasses #-}
    module B where

    import A

    instance A a where
      int _ = 1

    instance C a [b] where
      m _ = id

=========
    {-# LANGUAGE FlexibleContexts      #-}
    {-# LANGUAGE FlexibleInstances     #-}
    {-# LANGUAGE IncoherentInstances   #-}
    {-# LANGUAGE MultiParamTypeClasses #-}
    module C where

    import A

    instance A Int where
      int _ = 2

    instance C Int [Int] where
      m _ = id

    intC :: C Int a => a -> Int -> Int
    intC _ x = int x

=========
    module Main where

    import A
    import B
    import C

    main :: IO ()
    main = print (intC [] (0::Int))

The output of `main` if we avoid the optimization under the effect of
IncoherentInstances is `1`. If we were to do the optimization, the output of
`main` would be `2`.

Note [Shortcut try_solve_from_instance]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The workhorse of the short-cut solver is
    try_solve_from_instance :: (EvBindMap, DictMap CtEvidence)
                            -> CtEvidence       -- Solve this
                            -> MaybeT TcS (EvBindMap, DictMap CtEvidence)
Note that:

* The CtEvidence is the goal to be solved

* The MaybeT manages early failure if we find a subgoal that
  cannot be solved from instances.

* The (EvBindMap, DictMap CtEvidence) is an accumulating purely-functional
  state that allows try_solve_from_instance to augment the evidence
  bindings and inert_solved_dicts as it goes.

  If it succeeds, we commit all these bindings and solved dicts to the
  main TcS InertSet.  If not, we abandon it all entirely.

Passing along the solved_dicts important for two reasons:

* We need to be able to handle recursive super classes. The
  solved_dicts state  ensures that we remember what we have already
  tried to solve to avoid looping.

* As #15164 showed, it can be important to exploit sharing between
  goals. E.g. To solve G we may need G1 and G2. To solve G1 we may need H;
  and to solve G2 we may need H. If we don't spot this sharing we may
  solve H twice; and if this pattern repeats we may get exponentially bad
  behaviour.

Note [No Given/Given fundeps]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We do not create constraints from:
* Given/Given interactions via functional dependencies or type family
  injectivity annotations.
* Given/instance fundep interactions via functional dependencies or
  type family injectivity annotations.

In this Note, all these interactions are called just "fundeps".

We ingore such fundeps for several reasons:

1. These fundeps will never serve a purpose in accepting more
   programs: Given constraints do not contain metavariables that could
   be unified via exploring fundeps. They *could* be useful in
   discovering inaccessible code. However, the constraints will be
   Wanteds, and as such will cause errors (not just warnings) if they
   go unsolved. Maybe there is a clever way to get the right
   inaccessible code warnings, but the path forward is far from
   clear. #12466 has further commentary.

2. Furthermore, here is a case where a Given/instance interaction is actively
   harmful (from dependent/should_compile/RaeJobTalk):

       type family a == b :: Bool
       type family Not a = r | r -> a where
         Not False = True
         Not True  = False

       [G] Not (a == b) ~ True

   Reacting this Given with the equations for Not produces

      [W] a == b ~ False

   This is indeed a true consequence, and would make sense as a fresh Given.
   But we don't have a way to produce evidence for fundeps, as a Wanted it
   is /harmful/: we can't prove it, and so we'll report an error and reject
   the program. (Previously fundeps gave rise to Deriveds, which
   carried no evidence, so it didn't matter that they could not be proved.)

3. #20922 showed a subtle different problem with Given/instance fundeps.
      type family ZipCons (as :: [k]) (bssx :: [[k]]) = (r :: [[k]]) | r -> as bssx where
        ZipCons (a ': as) (bs ': bss) = (a ': bs) ': ZipCons as bss
        ...

      tclevel = 4
      [G] ZipCons is1 iss ~ (i : is2) : jss

   (The tclevel=4 means that this Given is at level 4.)  The fundep tells us that
   'iss' must be of form (is2 : beta[4]) where beta[4] is a fresh unification
   variable; we don't know what type it stands for. So we would emit
      [W] iss ~ is2 : beta

   Again we can't prove that equality; and worse we'll rewrite iss to
   (is2:beta) in deeply nested constraints inside this implication,
   where beta is untouchable (under other equality constraints), leading
   to other insoluble constraints.

The bottom line: since we have no evidence for them, we should ignore Given/Given
and Given/instance fundeps entirely.
-}

interactDict :: InertCans -> Ct -> TcS (StopOrContinue Ct)
interactDict :: InertCans -> SimplifierStage
interactDict InertCans
inerts ct_w :: Ct
ct_w@(CDictCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev_w, cc_class :: Ct -> Class
cc_class = Class
cls, cc_tyargs :: Ct -> [TcPredType]
cc_tyargs = [TcPredType]
tys })
  | Just Ct
ct_i <- InertCans -> CtLoc -> Class -> [TcPredType] -> Maybe Ct
lookupInertDict InertCans
inerts (CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev_w) Class
cls [TcPredType]
tys
  , let ev_i :: CtEvidence
ev_i  = Ct -> CtEvidence
ctEvidence Ct
ct_i
        loc_i :: CtLoc
loc_i = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev_i
        loc_w :: CtLoc
loc_w = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev_w
  = -- There is a matching dictionary in the inert set
    do { -- First to try to solve it /completely/ from top level instances
         -- See Note [Shortcut solving]
         DynFlags
dflags <- TcS DynFlags
forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
       ; Bool
short_cut_worked <- DynFlags -> CtEvidence -> CtEvidence -> TcS Bool
shortCutSolver DynFlags
dflags CtEvidence
ev_w CtEvidence
ev_i
       ; if Bool
short_cut_worked
         then CtEvidence -> String -> TcS (StopOrContinue Ct)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev_w String
"interactDict/solved from instance"

         -- Next see if we are in "loopy-superclass" land.  If so,
         -- we don't want to replace the (Given) inert with the
         -- (Wanted) work-item, or vice versa; we want to hang on
         -- to both, and try to solve the work-item via an instance.
         -- See Note [Solving superclass constraints] in GHC.Tc.TyCl.Instance
         else if CtLoc -> CtLoc -> Bool
prohibitedSuperClassSolve CtLoc
loc_i CtLoc
loc_w
         then SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
ct_w
         else
    do { -- The short-cut solver didn't fire, and loopy superclasses
         -- are dealt with, so we can either solve
         -- the inert from the work-item or vice-versa.
       ; case Ct -> Ct -> InteractResult
solveOneFromTheOther Ct
ct_i Ct
ct_w of
           InteractResult
KeepInert -> do { String -> SDoc -> TcS ()
traceTcS String
"lookupInertDict:KeepInert" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
ct_w)
                           ; CtEvidence -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev_w (CtEvidence -> EvTerm
ctEvTerm CtEvidence
ev_i)
                           ; StopOrContinue Ct -> TcS (StopOrContinue Ct)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (StopOrContinue Ct -> TcS (StopOrContinue Ct))
-> StopOrContinue Ct -> TcS (StopOrContinue Ct)
forall a b. (a -> b) -> a -> b
$ CtEvidence -> SDoc -> StopOrContinue Ct
forall a. CtEvidence -> SDoc -> StopOrContinue a
Stop CtEvidence
ev_w (String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Dict equal" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
ct_w) }
           InteractResult
KeepWork  -> do { String -> SDoc -> TcS ()
traceTcS String
"lookupInertDict:KeepWork" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
ct_w)
                           ; CtEvidence -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev_i (CtEvidence -> EvTerm
ctEvTerm CtEvidence
ev_w)
                           ; (DictMap Ct -> DictMap Ct) -> TcS ()
updInertDicts ((DictMap Ct -> DictMap Ct) -> TcS ())
-> (DictMap Ct -> DictMap Ct) -> TcS ()
forall a b. (a -> b) -> a -> b
$ \ DictMap Ct
ds -> DictMap Ct -> Class -> [TcPredType] -> DictMap Ct
forall a. DictMap a -> Class -> [TcPredType] -> DictMap a
delDict DictMap Ct
ds Class
cls [TcPredType]
tys
                           ; SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
ct_w } } }

  | Class
cls Class -> Unique -> Bool
forall a. Uniquable a => a -> Unique -> Bool
`hasKey` Unique
ipClassKey
  , CtEvidence -> Bool
isGiven CtEvidence
ev_w
  = InertCans -> SimplifierStage
interactGivenIP InertCans
inerts Ct
ct_w

  | Bool
otherwise
  = do { InertCans -> CtEvidence -> Class -> TcS ()
addFunDepWork InertCans
inerts CtEvidence
ev_w Class
cls
       ; SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
ct_w  }

interactDict InertCans
_ Ct
wi = String -> SDoc -> TcS (StopOrContinue Ct)
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"interactDict" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
wi)

-- See Note [Shortcut solving]
shortCutSolver :: DynFlags
               -> CtEvidence -- Work item
               -> CtEvidence -- Inert we want to try to replace
               -> TcS Bool   -- True <=> success
shortCutSolver :: DynFlags -> CtEvidence -> CtEvidence -> TcS Bool
shortCutSolver DynFlags
dflags CtEvidence
ev_w CtEvidence
ev_i
  | CtEvidence -> Bool
isWanted CtEvidence
ev_w
 Bool -> Bool -> Bool
&& CtEvidence -> Bool
isGiven CtEvidence
ev_i
 -- We are about to solve a [W] constraint from a [G] constraint. We take
 -- a moment to see if we can get a better solution using an instance.
 -- Note that we only do this for the sake of performance. Exactly the same
 -- programs should typecheck regardless of whether we take this step or
 -- not. See Note [Shortcut solving]

 Bool -> Bool -> Bool
&& Bool -> Bool
not (TcPredType -> Bool
isIPLikePred (CtEvidence -> TcPredType
ctEvPred CtEvidence
ev_w))   -- Not for implicit parameters (#18627)

 Bool -> Bool -> Bool
&& Bool -> Bool
not (Extension -> DynFlags -> Bool
xopt Extension
LangExt.IncoherentInstances DynFlags
dflags)
 -- If IncoherentInstances is on then we cannot rely on coherence of proofs
 -- in order to justify this optimization: The proof provided by the
 -- [G] constraint's superclass may be different from the top-level proof.
 -- See Note [Shortcut solving: incoherence]

 Bool -> Bool -> Bool
&& GeneralFlag -> DynFlags -> Bool
gopt GeneralFlag
Opt_SolveConstantDicts DynFlags
dflags
 -- Enabled by the -fsolve-constant-dicts flag

  = do { EvBindsVar
ev_binds_var <- TcS EvBindsVar
getTcEvBindsVar
       ; EvBindMap
ev_binds <- Bool -> SDoc -> TcS EvBindMap -> TcS EvBindMap
forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr (Bool -> Bool
not (EvBindsVar -> Bool
isCoEvBindsVar EvBindsVar
ev_binds_var )) (CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev_w) (TcS EvBindMap -> TcS EvBindMap) -> TcS EvBindMap -> TcS EvBindMap
forall a b. (a -> b) -> a -> b
$
                     EvBindsVar -> TcS EvBindMap
getTcEvBindsMap EvBindsVar
ev_binds_var
       ; DictMap CtEvidence
solved_dicts <- TcS (DictMap CtEvidence)
getSolvedDicts

       ; Maybe (EvBindMap, DictMap CtEvidence)
mb_stuff <- MaybeT TcS (EvBindMap, DictMap CtEvidence)
-> TcS (Maybe (EvBindMap, DictMap CtEvidence))
forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
runMaybeT (MaybeT TcS (EvBindMap, DictMap CtEvidence)
 -> TcS (Maybe (EvBindMap, DictMap CtEvidence)))
-> MaybeT TcS (EvBindMap, DictMap CtEvidence)
-> TcS (Maybe (EvBindMap, DictMap CtEvidence))
forall a b. (a -> b) -> a -> b
$ (EvBindMap, DictMap CtEvidence)
-> CtEvidence -> MaybeT TcS (EvBindMap, DictMap CtEvidence)
try_solve_from_instance
                                   (EvBindMap
ev_binds, DictMap CtEvidence
solved_dicts) CtEvidence
ev_w

       ; case Maybe (EvBindMap, DictMap CtEvidence)
mb_stuff of
           Maybe (EvBindMap, DictMap CtEvidence)
Nothing -> Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False
           Just (EvBindMap
ev_binds', DictMap CtEvidence
solved_dicts')
              -> do { EvBindsVar -> EvBindMap -> TcS ()
setTcEvBindsMap EvBindsVar
ev_binds_var EvBindMap
ev_binds'
                    ; DictMap CtEvidence -> TcS ()
setSolvedDicts DictMap CtEvidence
solved_dicts'
                    ; Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
True } }

  | Bool
otherwise
  = Bool -> TcS Bool
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
False
  where
    -- This `CtLoc` is used only to check the well-staged condition of any
    -- candidate DFun. Our subgoals all have the same stage as our root
    -- [W] constraint so it is safe to use this while solving them.
    loc_w :: CtLoc
loc_w = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev_w

    try_solve_from_instance   -- See Note [Shortcut try_solve_from_instance]
      :: (EvBindMap, DictMap CtEvidence) -> CtEvidence
      -> MaybeT TcS (EvBindMap, DictMap CtEvidence)
    try_solve_from_instance :: (EvBindMap, DictMap CtEvidence)
-> CtEvidence -> MaybeT TcS (EvBindMap, DictMap CtEvidence)
try_solve_from_instance (EvBindMap
ev_binds, DictMap CtEvidence
solved_dicts) CtEvidence
ev
      | let pred :: TcPredType
pred = CtEvidence -> TcPredType
ctEvPred CtEvidence
ev
            loc :: CtLoc
loc  = CtEvidence -> CtLoc
ctEvLoc  CtEvidence
ev
      , ClassPred Class
cls [TcPredType]
tys <- TcPredType -> Pred
classifyPredType TcPredType
pred
      = do { ClsInstResult
inst_res <- TcS ClsInstResult -> MaybeT TcS ClsInstResult
forall (m :: * -> *) a. Monad m => m a -> MaybeT m a
forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift (TcS ClsInstResult -> MaybeT TcS ClsInstResult)
-> TcS ClsInstResult -> MaybeT TcS ClsInstResult
forall a b. (a -> b) -> a -> b
$ DynFlags -> Bool -> Class -> [TcPredType] -> TcS ClsInstResult
matchGlobalInst DynFlags
dflags Bool
True Class
cls [TcPredType]
tys
           ; case ClsInstResult
inst_res of
               OneInst { cir_new_theta :: ClsInstResult -> [TcPredType]
cir_new_theta = [TcPredType]
preds
                       , cir_mk_ev :: ClsInstResult -> [EvExpr] -> EvTerm
cir_mk_ev     = [EvExpr] -> EvTerm
mk_ev
                       , cir_what :: ClsInstResult -> InstanceWhat
cir_what      = InstanceWhat
what }
                 | InstanceWhat -> Bool
safeOverlap InstanceWhat
what
                 , (TcPredType -> Bool) -> [TcPredType] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
all TcPredType -> Bool
isTyFamFree [TcPredType]
preds  -- Note [Shortcut solving: type families]
                 -> do { let solved_dicts' :: DictMap CtEvidence
solved_dicts' = DictMap CtEvidence
-> Class -> [TcPredType] -> CtEvidence -> DictMap CtEvidence
forall a. DictMap a -> Class -> [TcPredType] -> a -> DictMap a
addDict DictMap CtEvidence
solved_dicts Class
cls [TcPredType]
tys CtEvidence
ev
                             -- solved_dicts': it is important that we add our goal
                             -- to the cache before we solve! Otherwise we may end
                             -- up in a loop while solving recursive dictionaries.

                       ; TcS () -> MaybeT TcS ()
forall (m :: * -> *) a. Monad m => m a -> MaybeT m a
forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift (TcS () -> MaybeT TcS ()) -> TcS () -> MaybeT TcS ()
forall a b. (a -> b) -> a -> b
$ String -> SDoc -> TcS ()
traceTcS String
"shortCutSolver: found instance" ([TcPredType] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TcPredType]
preds)
                       ; CtLoc
loc' <- TcS CtLoc -> MaybeT TcS CtLoc
forall (m :: * -> *) a. Monad m => m a -> MaybeT m a
forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift (TcS CtLoc -> MaybeT TcS CtLoc) -> TcS CtLoc -> MaybeT TcS CtLoc
forall a b. (a -> b) -> a -> b
$ CtLoc -> InstanceWhat -> TcPredType -> TcS CtLoc
checkInstanceOK CtLoc
loc InstanceWhat
what TcPredType
pred
                       ; TcS () -> MaybeT TcS ()
forall (m :: * -> *) a. Monad m => m a -> MaybeT m a
forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift (TcS () -> MaybeT TcS ()) -> TcS () -> MaybeT TcS ()
forall a b. (a -> b) -> a -> b
$ CtLoc -> TcPredType -> TcS ()
checkReductionDepth CtLoc
loc' TcPredType
pred


                       ; [MaybeNew]
evc_vs <- (TcPredType -> MaybeT TcS MaybeNew)
-> [TcPredType] -> MaybeT TcS [MaybeNew]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
mapM (CtEvidence
-> CtLoc -> DictMap CtEvidence -> TcPredType -> MaybeT TcS MaybeNew
new_wanted_cached CtEvidence
ev CtLoc
loc' DictMap CtEvidence
solved_dicts') [TcPredType]
preds
                                  -- Emit work for subgoals but use our local cache
                                  -- so we can solve recursive dictionaries.

                       ; let ev_tm :: EvTerm
ev_tm     = [EvExpr] -> EvTerm
mk_ev ((MaybeNew -> EvExpr) -> [MaybeNew] -> [EvExpr]
forall a b. (a -> b) -> [a] -> [b]
map MaybeNew -> EvExpr
getEvExpr [MaybeNew]
evc_vs)
                             ev_binds' :: EvBindMap
ev_binds' = EvBindMap -> EvBind -> EvBindMap
extendEvBinds EvBindMap
ev_binds (EvBind -> EvBindMap) -> EvBind -> EvBindMap
forall a b. (a -> b) -> a -> b
$
                                         TyVar -> EvTerm -> EvBind
mkWantedEvBind (CtEvidence -> TyVar
ctEvEvId CtEvidence
ev) EvTerm
ev_tm

                       ; ((EvBindMap, DictMap CtEvidence)
 -> CtEvidence -> MaybeT TcS (EvBindMap, DictMap CtEvidence))
-> (EvBindMap, DictMap CtEvidence)
-> [CtEvidence]
-> MaybeT TcS (EvBindMap, DictMap CtEvidence)
forall (t :: * -> *) (m :: * -> *) b a.
(Foldable t, Monad m) =>
(b -> a -> m b) -> b -> t a -> m b
foldlM (EvBindMap, DictMap CtEvidence)
-> CtEvidence -> MaybeT TcS (EvBindMap, DictMap CtEvidence)
try_solve_from_instance
                                (EvBindMap
ev_binds', DictMap CtEvidence
solved_dicts')
                                ([MaybeNew] -> [CtEvidence]
freshGoals [MaybeNew]
evc_vs) }

               ClsInstResult
_ -> MaybeT TcS (EvBindMap, DictMap CtEvidence)
forall a. MaybeT TcS a
forall (m :: * -> *) a. MonadPlus m => m a
mzero }
      | Bool
otherwise = MaybeT TcS (EvBindMap, DictMap CtEvidence)
forall a. MaybeT TcS a
forall (m :: * -> *) a. MonadPlus m => m a
mzero


    -- Use a local cache of solved dicts while emitting EvVars for new work
    -- We bail out of the entire computation if we need to emit an EvVar for
    -- a subgoal that isn't a ClassPred.
    new_wanted_cached :: CtEvidence -> CtLoc
                      -> DictMap CtEvidence -> TcPredType -> MaybeT TcS MaybeNew
    new_wanted_cached :: CtEvidence
-> CtLoc -> DictMap CtEvidence -> TcPredType -> MaybeT TcS MaybeNew
new_wanted_cached CtEvidence
ev_w CtLoc
loc DictMap CtEvidence
cache TcPredType
pty
      | ClassPred Class
cls [TcPredType]
tys <- TcPredType -> Pred
classifyPredType TcPredType
pty
      = TcS MaybeNew -> MaybeT TcS MaybeNew
forall (m :: * -> *) a. Monad m => m a -> MaybeT m a
forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift (TcS MaybeNew -> MaybeT TcS MaybeNew)
-> TcS MaybeNew -> MaybeT TcS MaybeNew
forall a b. (a -> b) -> a -> b
$ case DictMap CtEvidence
-> CtLoc -> Class -> [TcPredType] -> Maybe CtEvidence
forall a. DictMap a -> CtLoc -> Class -> [TcPredType] -> Maybe a
findDict DictMap CtEvidence
cache CtLoc
loc_w Class
cls [TcPredType]
tys of
          Just CtEvidence
ctev -> MaybeNew -> TcS MaybeNew
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (MaybeNew -> TcS MaybeNew) -> MaybeNew -> TcS MaybeNew
forall a b. (a -> b) -> a -> b
$ EvExpr -> MaybeNew
Cached ((() :: Constraint) => CtEvidence -> EvExpr
CtEvidence -> EvExpr
ctEvExpr CtEvidence
ctev)
          Maybe CtEvidence
Nothing   -> CtEvidence -> MaybeNew
Fresh (CtEvidence -> MaybeNew) -> TcS CtEvidence -> TcS MaybeNew
forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> CtLoc -> RewriterSet -> TcPredType -> TcS CtEvidence
newWantedNC CtLoc
loc (CtEvidence -> RewriterSet
ctEvRewriters CtEvidence
ev_w) TcPredType
pty
      | Bool
otherwise = MaybeT TcS MaybeNew
forall a. MaybeT TcS a
forall (m :: * -> *) a. MonadPlus m => m a
mzero

addFunDepWork :: InertCans -> CtEvidence -> Class -> TcS ()
-- Add wanted constraints from type-class functional dependencies.
addFunDepWork :: InertCans -> CtEvidence -> Class -> TcS ()
addFunDepWork InertCans
inerts CtEvidence
work_ev Class
cls
  = (Ct -> TcS ()) -> Cts -> TcS ()
forall (m :: * -> *) a b. Monad m => (a -> m b) -> Bag a -> m ()
mapBagM_ Ct -> TcS ()
add_fds (DictMap Ct -> Class -> Cts
forall a. DictMap a -> Class -> Bag a
findDictsByClass (InertCans -> DictMap Ct
inert_dicts InertCans
inerts) Class
cls)
               -- No need to check flavour; fundeps work between
               -- any pair of constraints, regardless of flavour
               -- Importantly we don't throw workitem back in the
               -- worklist because this can cause loops (see #5236)
  where
    work_pred :: TcPredType
work_pred = CtEvidence -> TcPredType
ctEvPred CtEvidence
work_ev
    work_loc :: CtLoc
work_loc  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
work_ev

    add_fds :: Ct -> TcS ()
add_fds Ct
inert_ct
      = do { String -> SDoc -> TcS ()
traceTcS String
"addFunDepWork" ([SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat
                [ CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
work_ev
                , CtLoc -> SDoc
pprCtLoc CtLoc
work_loc, Bool -> SDoc
forall a. Outputable a => a -> SDoc
ppr (CtLoc -> Bool
isGivenLoc CtLoc
work_loc)
                , CtLoc -> SDoc
pprCtLoc CtLoc
inert_loc, Bool -> SDoc
forall a. Outputable a => a -> SDoc
ppr (CtLoc -> Bool
isGivenLoc CtLoc
inert_loc)
                , CtLoc -> SDoc
pprCtLoc CtLoc
derived_loc, Bool -> SDoc
forall a. Outputable a => a -> SDoc
ppr (CtLoc -> Bool
isGivenLoc CtLoc
derived_loc) ])

           ; Bool -> TcS () -> TcS ()
forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
unless (CtEvidence -> Bool
isGiven CtEvidence
work_ev Bool -> Bool -> Bool
&& CtEvidence -> Bool
isGiven CtEvidence
inert_ev) (TcS () -> TcS ()) -> TcS () -> TcS ()
forall a b. (a -> b) -> a -> b
$
             RewriterSet -> [FunDepEqn (CtLoc, RewriterSet)] -> TcS ()
emitFunDepWanteds (CtEvidence -> RewriterSet
ctEvRewriters CtEvidence
work_ev) ([FunDepEqn (CtLoc, RewriterSet)] -> TcS ())
-> [FunDepEqn (CtLoc, RewriterSet)] -> TcS ()
forall a b. (a -> b) -> a -> b
$
             (CtLoc, RewriterSet)
-> TcPredType -> TcPredType -> [FunDepEqn (CtLoc, RewriterSet)]
forall loc. loc -> TcPredType -> TcPredType -> [FunDepEqn loc]
improveFromAnother (CtLoc
derived_loc, RewriterSet
inert_rewriters) TcPredType
inert_pred TcPredType
work_pred
               -- We don't really rewrite tys2, see below _rewritten_tys2, so that's ok
               -- Do not create FDs from Given/Given interactions: See Note [No Given/Given fundeps]
        }
      where
        inert_ev :: CtEvidence
inert_ev   = Ct -> CtEvidence
ctEvidence Ct
inert_ct
        inert_pred :: TcPredType
inert_pred = CtEvidence -> TcPredType
ctEvPred CtEvidence
inert_ev
        inert_loc :: CtLoc
inert_loc  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
inert_ev
        inert_rewriters :: RewriterSet
inert_rewriters = Ct -> RewriterSet
ctRewriters Ct
inert_ct
        derived_loc :: CtLoc
derived_loc = CtLoc
work_loc { ctl_depth  = ctl_depth work_loc `maxSubGoalDepth`
                                              ctl_depth inert_loc
                               , ctl_origin = FunDepOrigin1 work_pred
                                                            (ctLocOrigin work_loc)
                                                            (ctLocSpan work_loc)
                                                            inert_pred
                                                            (ctLocOrigin inert_loc)
                                                            (ctLocSpan inert_loc) }

{-
**********************************************************************
*                                                                    *
                   Implicit parameters
*                                                                    *
**********************************************************************
-}

interactGivenIP :: InertCans -> Ct -> TcS (StopOrContinue Ct)
-- Work item is Given (?x:ty)
-- See Note [Shadowing of Implicit Parameters]
interactGivenIP :: InertCans -> SimplifierStage
interactGivenIP InertCans
inerts workItem :: Ct
workItem@(CDictCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev, cc_class :: Ct -> Class
cc_class = Class
cls
                                          , cc_tyargs :: Ct -> [TcPredType]
cc_tyargs = tys :: [TcPredType]
tys@(TcPredType
ip_str:[TcPredType]
_) })
  = do { (InertCans -> InertCans) -> TcS ()
updInertCans ((InertCans -> InertCans) -> TcS ())
-> (InertCans -> InertCans) -> TcS ()
forall a b. (a -> b) -> a -> b
$ \InertCans
cans -> InertCans
cans { inert_dicts = addDict filtered_dicts cls tys workItem }
       ; CtEvidence -> String -> TcS (StopOrContinue Ct)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Given IP" }
  where
    dicts :: DictMap Ct
dicts           = InertCans -> DictMap Ct
inert_dicts InertCans
inerts
    ip_dicts :: Cts
ip_dicts        = DictMap Ct -> Class -> Cts
forall a. DictMap a -> Class -> Bag a
findDictsByClass DictMap Ct
dicts Class
cls
    other_ip_dicts :: Cts
other_ip_dicts  = (Ct -> Bool) -> Cts -> Cts
forall a. (a -> Bool) -> Bag a -> Bag a
filterBag (Bool -> Bool
not (Bool -> Bool) -> (Ct -> Bool) -> Ct -> Bool
forall b c a. (b -> c) -> (a -> b) -> a -> c
. Ct -> Bool
is_this_ip) Cts
ip_dicts
    filtered_dicts :: DictMap Ct
filtered_dicts  = DictMap Ct -> Class -> Cts -> DictMap Ct
addDictsByClass DictMap Ct
dicts Class
cls Cts
other_ip_dicts

    -- Pick out any Given constraints for the same implicit parameter
    is_this_ip :: Ct -> Bool
is_this_ip (CDictCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev, cc_tyargs :: Ct -> [TcPredType]
cc_tyargs = TcPredType
ip_str':[TcPredType]
_ })
       = CtEvidence -> Bool
isGiven CtEvidence
ev Bool -> Bool -> Bool
&& TcPredType
ip_str (() :: Constraint) => TcPredType -> TcPredType -> Bool
TcPredType -> TcPredType -> Bool
`tcEqType` TcPredType
ip_str'
    is_this_ip Ct
_ = Bool
False

interactGivenIP InertCans
_ Ct
wi = String -> SDoc -> TcS (StopOrContinue Ct)
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"interactGivenIP" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
wi)

{- Note [Shadowing of Implicit Parameters]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the following example:

f :: (?x :: Char) => Char
f = let ?x = 'a' in ?x

The "let ?x = ..." generates an implication constraint of the form:

?x :: Char => ?x :: Char

Furthermore, the signature for `f` also generates an implication
constraint, so we end up with the following nested implication:

?x :: Char => (?x :: Char => ?x :: Char)

Note that the wanted (?x :: Char) constraint may be solved in
two incompatible ways:  either by using the parameter from the
signature, or by using the local definition.  Our intention is
that the local definition should "shadow" the parameter of the
signature, and we implement this as follows: when we add a new
*given* implicit parameter to the inert set, it replaces any existing
givens for the same implicit parameter.

Similarly, consider
   f :: (?x::a) => Bool -> a

   g v = let ?x::Int = 3
         in (f v, let ?x::Bool = True in f v)

This should probably be well typed, with
   g :: Bool -> (Int, Bool)

So the inner binding for ?x::Bool *overrides* the outer one.

See ticket #17104 for a rather tricky example of this overriding
behaviour.

All this works for the normal cases but it has an odd side effect in
some pathological programs like this:
-- This is accepted, the second parameter shadows
f1 :: (?x :: Int, ?x :: Char) => Char
f1 = ?x

-- This is rejected, the second parameter shadows
f2 :: (?x :: Int, ?x :: Char) => Int
f2 = ?x

Both of these are actually wrong:  when we try to use either one,
we'll get two incompatible wanted constraints (?x :: Int, ?x :: Char),
which would lead to an error.

I can think of two ways to fix this:

  1. Simply disallow multiple constraints for the same implicit
    parameter---this is never useful, and it can be detected completely
    syntactically.

  2. Move the shadowing machinery to the location where we nest
     implications, and add some code here that will produce an
     error if we get multiple givens for the same implicit parameter.


**********************************************************************
*                                                                    *
                   interactFunEq
*                                                                    *
**********************************************************************
-}

improveLocalFunEqs :: CtEvidence -> InertCans -> TyCon -> [TcType] -> TcType
                   -> TcS ()
-- Generate improvement equalities, by comparing
-- the current work item with inert CFunEqs
-- E.g.   x + y ~ z,   x + y' ~ z   =>   [W] y ~ y'
--
-- See Note [FunDep and implicit parameter reactions]
improveLocalFunEqs :: CtEvidence
-> InertCans -> TyCon -> [TcPredType] -> TcPredType -> TcS ()
improveLocalFunEqs CtEvidence
work_ev InertCans
inerts TyCon
fam_tc [TcPredType]
args TcPredType
rhs
  = Bool -> TcS () -> TcS ()
forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
unless ([FunDepEqn (CtLoc, RewriterSet)] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [FunDepEqn (CtLoc, RewriterSet)]
improvement_eqns) (TcS () -> TcS ()) -> TcS () -> TcS ()
forall a b. (a -> b) -> a -> b
$
    do { String -> SDoc -> TcS ()
traceTcS String
"interactFunEq improvements: " (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
                   [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Eqns:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [FunDepEqn (CtLoc, RewriterSet)] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [FunDepEqn (CtLoc, RewriterSet)]
improvement_eqns
                        , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Candidates:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [Ct] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [Ct]
funeqs_for_tc
                        , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Inert eqs:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> InertEqs -> SDoc
forall a. Outputable a => a -> SDoc
ppr (InertCans -> InertEqs
inert_eqs InertCans
inerts) ]
       ; RewriterSet -> [FunDepEqn (CtLoc, RewriterSet)] -> TcS ()
emitFunDepWanteds (CtEvidence -> RewriterSet
ctEvRewriters CtEvidence
work_ev) [FunDepEqn (CtLoc, RewriterSet)]
improvement_eqns }
  where
    funeqs :: FunEqMap [Ct]
funeqs        = InertCans -> FunEqMap [Ct]
inert_funeqs InertCans
inerts
    funeqs_for_tc :: [Ct]
funeqs_for_tc = [ Ct
funeq_ct | [Ct]
equal_ct_list <- FunEqMap [Ct] -> TyCon -> [[Ct]]
forall a. FunEqMap a -> TyCon -> [a]
findFunEqsByTyCon FunEqMap [Ct]
funeqs TyCon
fam_tc
                               , Ct
funeq_ct <- [Ct]
equal_ct_list
                               , EqRel
NomEq EqRel -> EqRel -> Bool
forall a. Eq a => a -> a -> Bool
== Ct -> EqRel
ctEqRel Ct
funeq_ct ]
                                  -- representational equalities don't interact
                                  -- with type family dependencies
    work_loc :: CtLoc
work_loc      = CtEvidence -> CtLoc
ctEvLoc CtEvidence
work_ev
    work_pred :: TcPredType
work_pred     = CtEvidence -> TcPredType
ctEvPred CtEvidence
work_ev
    fam_inj_info :: Injectivity
fam_inj_info  = TyCon -> Injectivity
tyConInjectivityInfo TyCon
fam_tc

    --------------------
    improvement_eqns :: [FunDepEqn (CtLoc, RewriterSet)]
    improvement_eqns :: [FunDepEqn (CtLoc, RewriterSet)]
improvement_eqns
      | Just BuiltInSynFamily
ops <- TyCon -> Maybe BuiltInSynFamily
isBuiltInSynFamTyCon_maybe TyCon
fam_tc
      =    -- Try built-in families, notably for arithmethic
        (Ct -> [FunDepEqn (CtLoc, RewriterSet)])
-> [Ct] -> [FunDepEqn (CtLoc, RewriterSet)]
forall (t :: * -> *) a b. Foldable t => (a -> [b]) -> t a -> [b]
concatMap (BuiltInSynFamily
-> TcPredType -> Ct -> [FunDepEqn (CtLoc, RewriterSet)]
do_one_built_in BuiltInSynFamily
ops TcPredType
rhs) [Ct]
funeqs_for_tc

      | Injective [Bool]
injective_args <- Injectivity
fam_inj_info
      =    -- Try improvement from type families with injectivity annotations
        (Ct -> [FunDepEqn (CtLoc, RewriterSet)])
-> [Ct] -> [FunDepEqn (CtLoc, RewriterSet)]
forall (t :: * -> *) a b. Foldable t => (a -> [b]) -> t a -> [b]
concatMap ([Bool] -> TcPredType -> Ct -> [FunDepEqn (CtLoc, RewriterSet)]
do_one_injective [Bool]
injective_args TcPredType
rhs) [Ct]
funeqs_for_tc

      | Bool
otherwise
      = []

    --------------------
    do_one_built_in :: BuiltInSynFamily
-> TcPredType -> Ct -> [FunDepEqn (CtLoc, RewriterSet)]
do_one_built_in BuiltInSynFamily
ops TcPredType
rhs (CEqCan { cc_lhs :: Ct -> CanEqLHS
cc_lhs = TyFamLHS TyCon
_ [TcPredType]
iargs, cc_rhs :: Ct -> TcPredType
cc_rhs = TcPredType
irhs, cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
inert_ev })
      | Bool -> Bool
not (CtEvidence -> Bool
isGiven CtEvidence
inert_ev Bool -> Bool -> Bool
&& CtEvidence -> Bool
isGiven CtEvidence
work_ev)  -- See Note [No Given/Given fundeps]
      = CtEvidence -> [TypeEqn] -> [FunDepEqn (CtLoc, RewriterSet)]
mk_fd_eqns CtEvidence
inert_ev (BuiltInSynFamily
-> [TcPredType]
-> TcPredType
-> [TcPredType]
-> TcPredType
-> [TypeEqn]
sfInteractInert BuiltInSynFamily
ops [TcPredType]
args TcPredType
rhs [TcPredType]
iargs TcPredType
irhs)

      | Bool
otherwise
      = []

    do_one_built_in BuiltInSynFamily
_ TcPredType
_ Ct
_ = String -> SDoc -> [FunDepEqn (CtLoc, RewriterSet)]
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"interactFunEq 1" (TyCon -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyCon
fam_tc)

    --------------------
    -- See Note [Type inference for type families with injectivity]
    do_one_injective :: [Bool] -> TcPredType -> Ct -> [FunDepEqn (CtLoc, RewriterSet)]
do_one_injective [Bool]
inj_args TcPredType
rhs (CEqCan { cc_lhs :: Ct -> CanEqLHS
cc_lhs = TyFamLHS TyCon
_ [TcPredType]
inert_args
                                          , cc_rhs :: Ct -> TcPredType
cc_rhs = TcPredType
irhs, cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
inert_ev })
      | Bool -> Bool
not (CtEvidence -> Bool
isGiven CtEvidence
inert_ev Bool -> Bool -> Bool
&& CtEvidence -> Bool
isGiven CtEvidence
work_ev) -- See Note [No Given/Given fundeps]
      , TcPredType
rhs (() :: Constraint) => TcPredType -> TcPredType -> Bool
TcPredType -> TcPredType -> Bool
`tcEqType` TcPredType
irhs
      = CtEvidence -> [TypeEqn] -> [FunDepEqn (CtLoc, RewriterSet)]
mk_fd_eqns CtEvidence
inert_ev ([TypeEqn] -> [FunDepEqn (CtLoc, RewriterSet)])
-> [TypeEqn] -> [FunDepEqn (CtLoc, RewriterSet)]
forall a b. (a -> b) -> a -> b
$ [ TcPredType -> TcPredType -> TypeEqn
forall a. a -> a -> Pair a
Pair TcPredType
arg TcPredType
iarg
                              | (TcPredType
arg, TcPredType
iarg, Bool
True) <- [TcPredType]
-> [TcPredType] -> [Bool] -> [(TcPredType, TcPredType, Bool)]
forall a b c. [a] -> [b] -> [c] -> [(a, b, c)]
zip3 [TcPredType]
args [TcPredType]
inert_args [Bool]
inj_args ]
      | Bool
otherwise
      = []

    do_one_injective [Bool]
_ TcPredType
_ Ct
_ = String -> SDoc -> [FunDepEqn (CtLoc, RewriterSet)]
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"interactFunEq 2" (TyCon -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyCon
fam_tc)

    --------------------
    mk_fd_eqns :: CtEvidence -> [TypeEqn] -> [FunDepEqn (CtLoc, RewriterSet)]
    mk_fd_eqns :: CtEvidence -> [TypeEqn] -> [FunDepEqn (CtLoc, RewriterSet)]
mk_fd_eqns CtEvidence
inert_ev [TypeEqn]
eqns
      | [TypeEqn] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TypeEqn]
eqns  = []
      | Bool
otherwise  = [ FDEqn { fd_qtvs :: [TyVar]
fd_qtvs = [], fd_eqs :: [TypeEqn]
fd_eqs = [TypeEqn]
eqns
                             , fd_pred1 :: TcPredType
fd_pred1 = TcPredType
work_pred
                             , fd_pred2 :: TcPredType
fd_pred2 = TcPredType
inert_pred
                             , fd_loc :: (CtLoc, RewriterSet)
fd_loc   = (CtLoc
loc, RewriterSet
inert_rewriters) } ]
      where
        initial_loc :: CtLoc
initial_loc  -- start with the location of the Wanted involved
          | CtEvidence -> Bool
isGiven CtEvidence
work_ev = CtLoc
inert_loc
          | Bool
otherwise       = CtLoc
work_loc
        eqn_orig :: CtOrigin
eqn_orig        = TcPredType
-> CtOrigin
-> RealSrcSpan
-> TcPredType
-> CtOrigin
-> RealSrcSpan
-> CtOrigin
InjTFOrigin1 TcPredType
work_pred (CtLoc -> CtOrigin
ctLocOrigin CtLoc
work_loc) (CtLoc -> RealSrcSpan
ctLocSpan CtLoc
work_loc)
                                       TcPredType
inert_pred (CtLoc -> CtOrigin
ctLocOrigin CtLoc
inert_loc) (CtLoc -> RealSrcSpan
ctLocSpan CtLoc
inert_loc)
        eqn_loc :: CtLoc
eqn_loc         = CtLoc -> CtOrigin -> CtLoc
setCtLocOrigin CtLoc
initial_loc CtOrigin
eqn_orig
        inert_pred :: TcPredType
inert_pred      = CtEvidence -> TcPredType
ctEvPred CtEvidence
inert_ev
        inert_loc :: CtLoc
inert_loc       = CtEvidence -> CtLoc
ctEvLoc CtEvidence
inert_ev
        inert_rewriters :: RewriterSet
inert_rewriters = CtEvidence -> RewriterSet
ctEvRewriters CtEvidence
inert_ev
        loc :: CtLoc
loc = CtLoc
eqn_loc { ctl_depth = ctl_depth inert_loc `maxSubGoalDepth`
                                    ctl_depth work_loc }

{- Note [Type inference for type families with injectivity]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have a type family with an injectivity annotation:
    type family F a b = r | r -> b

Then if we have an equality like F s1 t1 ~ F s2 t2,
we can use the injectivity to get a new Wanted constraint on
the injective argument
  [W] t1 ~ t2

That in turn can help GHC solve constraints that would otherwise require
guessing.  For example, consider the ambiguity check for
   f :: F Int b -> Int
We get the constraint
   [W] F Int b ~ F Int beta
where beta is a unification variable.  Injectivity lets us pick beta ~ b.

Injectivity information is also used at the call sites. For example:
   g = f True
gives rise to
   [W] F Int b ~ Bool
from which we can derive b.  This requires looking at the defining equations of
a type family, ie. finding equation with a matching RHS (Bool in this example)
and inferring values of type variables (b in this example) from the LHS patterns
of the matching equation.  For closed type families we have to perform
additional apartness check for the selected equation to check that the selected
is guaranteed to fire for given LHS arguments.

These new constraints are Wanted constraints, but we will not use the evidence.
We could go further and offer evidence from decomposing injective type-function
applications, but that would require new evidence forms, and an extension to
FC, so we don't do that right now (Dec 14).

We generate these Wanteds in three places, depending on how we notice the
injectivity.

1. When we have a [W] F tys1 ~ F tys2. This is handled in canEqCanLHS2, and
described in Note [Decomposing type family applications] in GHC.Tc.Solver.Canonical.

2. When we have [W] F tys1 ~ T and [W] F tys2 ~ T. Note that neither of these
constraints rewrites the other, as they have different LHSs. This is done
in improveLocalFunEqs, called during the interactWithInertsStage.

3. When we have [W] F tys ~ T and an equation for F that looks like F tys' = T.
This is done in improve_top_fun_eqs, called from the top-level reactions stage.

See also Note [Injective type families] in GHC.Core.TyCon

Note [Cache-caused loops]
~~~~~~~~~~~~~~~~~~~~~~~~~
It is very dangerous to cache a rewritten wanted family equation as 'solved' in our
solved cache (which is the default behaviour or xCtEvidence), because the interaction
may not be contributing towards a solution. Here is an example:

Initial inert set:
  [W] g1 : F a ~ beta1
Work item:
  [W] g2 : F a ~ beta2
The work item will react with the inert yielding the _same_ inert set plus:
    (i)   Will set g2 := g1 `cast` g3
    (ii)  Will add to our solved cache that [S] g2 : F a ~ beta2
    (iii) Will emit [W] g3 : beta1 ~ beta2
Now, the g3 work item will be spontaneously solved to [G] g3 : beta1 ~ beta2
and then it will react the item in the inert ([W] g1 : F a ~ beta1). So it
will set
      g1 := g ; sym g3
and what is g? Well it would ideally be a new goal of type (F a ~ beta2) but
remember that we have this in our solved cache, and it is ... g2! In short we
created the evidence loop:

        g2 := g1 ; g3
        g3 := refl
        g1 := g2 ; sym g3

To avoid this situation we do not cache as solved any workitems (or inert)
which did not really made a 'step' towards proving some goal. Solved's are
just an optimization so we don't lose anything in terms of completeness of
solving.

**********************************************************************
*                                                                    *
                   interactEq
*                                                                    *
**********************************************************************
-}

{- Note [Combining equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have
   Inert:     g1 :: a ~ t
   Work item: g2 :: a ~ t

Then we can simply solve g2 from g1, thus g2 := g1.  Easy!
But it's not so simple:

* If t is a type variable, the equalties might be oriented differently:
      e.g. (g1 :: a~b) and (g2 :: b~a)
  So we look both ways round.  Hence the SwapFlag result to
  inertsCanDischarge.

* We can only do g2 := g1 if g1 can discharge g2; that depends on
  (a) the role and (b) the flavour.  E.g. a representational equality
  cannot discharge a nominal one; a Wanted cannot discharge a Given.
  The predicate is eqCanRewriteFR.

* Visibility. Suppose  S :: forall k. k -> Type, and consider unifying
      S @Type (a::Type)  ~   S @(Type->Type) (b::Type->Type)
  From the first argument we get (Type ~ Type->Type); from the second
  argument we get (a ~ b) which in turn gives (Type ~ Type->Type).
  See typecheck/should_fail/T16204c.

  That first argument is invisible in the source program (aside from
  visible type application), so we'd much prefer to get the error from
  the second. We track visibility in the uo_visible field of a TypeEqOrigin.
  We use this to prioritise visible errors (see GHC.Tc.Errors.tryReporters,
  the partition on isVisibleOrigin).

  So when combining two otherwise-identical equalites, we want to
  keep the visible one, and discharge the invisible one.  Hence the
  call to strictly_more_visible.
-}

inertsCanDischarge :: InertCans -> Ct
                   -> Maybe ( CtEvidence  -- The evidence for the inert
                            , SwapFlag )  -- Whether we need mkSymCo
inertsCanDischarge :: InertCans -> Ct -> Maybe (CtEvidence, SwapFlag)
inertsCanDischarge InertCans
inerts (CEqCan { cc_lhs :: Ct -> CanEqLHS
cc_lhs = CanEqLHS
lhs_w, cc_rhs :: Ct -> TcPredType
cc_rhs = TcPredType
rhs_w
                                  , cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev_w, cc_eq_rel :: Ct -> EqRel
cc_eq_rel = EqRel
eq_rel })
  | (CtEvidence
ev_i : [CtEvidence]
_) <- [ CtEvidence
ev_i | CEqCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev_i, cc_rhs :: Ct -> TcPredType
cc_rhs = TcPredType
rhs_i
                                  , cc_eq_rel :: Ct -> EqRel
cc_eq_rel = EqRel
eq_rel }
                             <- InertCans -> CanEqLHS -> [Ct]
findEq InertCans
inerts CanEqLHS
lhs_w
                         , TcPredType
rhs_i (() :: Constraint) => TcPredType -> TcPredType -> Bool
TcPredType -> TcPredType -> Bool
`tcEqType` TcPredType
rhs_w
                         , CtEvidence -> EqRel -> Bool
inert_beats_wanted CtEvidence
ev_i EqRel
eq_rel ]
  =  -- Inert:     a ~ ty
     -- Work item: a ~ ty
    (CtEvidence, SwapFlag) -> Maybe (CtEvidence, SwapFlag)
forall a. a -> Maybe a
Just (CtEvidence
ev_i, SwapFlag
NotSwapped)

  | Just CanEqLHS
rhs_lhs <- TcPredType -> Maybe CanEqLHS
canEqLHS_maybe TcPredType
rhs_w
  , (CtEvidence
ev_i : [CtEvidence]
_) <- [ CtEvidence
ev_i | CEqCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev_i, cc_rhs :: Ct -> TcPredType
cc_rhs = TcPredType
rhs_i
                                  , cc_eq_rel :: Ct -> EqRel
cc_eq_rel = EqRel
eq_rel }
                             <- InertCans -> CanEqLHS -> [Ct]
findEq InertCans
inerts CanEqLHS
rhs_lhs
                         , TcPredType
rhs_i (() :: Constraint) => TcPredType -> TcPredType -> Bool
TcPredType -> TcPredType -> Bool
`tcEqType` CanEqLHS -> TcPredType
canEqLHSType CanEqLHS
lhs_w
                         , CtEvidence -> EqRel -> Bool
inert_beats_wanted CtEvidence
ev_i EqRel
eq_rel ]
  =  -- Inert:     a ~ b
     -- Work item: b ~ a
     (CtEvidence, SwapFlag) -> Maybe (CtEvidence, SwapFlag)
forall a. a -> Maybe a
Just (CtEvidence
ev_i, SwapFlag
IsSwapped)

  where
    loc_w :: CtLoc
loc_w  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev_w
    flav_w :: CtFlavour
flav_w = CtEvidence -> CtFlavour
ctEvFlavour CtEvidence
ev_w
    fr_w :: (CtFlavour, EqRel)
fr_w   = (CtFlavour
flav_w, EqRel
eq_rel)

    inert_beats_wanted :: CtEvidence -> EqRel -> Bool
inert_beats_wanted CtEvidence
ev_i EqRel
eq_rel
      = -- eqCanRewriteFR:        see second bullet of Note [Combining equalities]
        -- strictly_more_visible: see last bullet of Note [Combining equalities]
        (CtFlavour, EqRel)
fr_i (CtFlavour, EqRel) -> (CtFlavour, EqRel) -> Bool
`eqCanRewriteFR` (CtFlavour, EqRel)
fr_w
        Bool -> Bool -> Bool
&& Bool -> Bool
not ((CtLoc
loc_w CtLoc -> CtLoc -> Bool
`strictly_more_visible` CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev_i)
                 Bool -> Bool -> Bool
&& ((CtFlavour, EqRel)
fr_w (CtFlavour, EqRel) -> (CtFlavour, EqRel) -> Bool
`eqCanRewriteFR` (CtFlavour, EqRel)
fr_i))
      where
        fr_i :: (CtFlavour, EqRel)
fr_i = (CtEvidence -> CtFlavour
ctEvFlavour CtEvidence
ev_i, EqRel
eq_rel)

    -- See Note [Combining equalities], final bullet
    strictly_more_visible :: CtLoc -> CtLoc -> Bool
strictly_more_visible CtLoc
loc1 CtLoc
loc2
       = Bool -> Bool
not (CtOrigin -> Bool
isVisibleOrigin (CtLoc -> CtOrigin
ctLocOrigin CtLoc
loc2)) Bool -> Bool -> Bool
&&
         CtOrigin -> Bool
isVisibleOrigin (CtLoc -> CtOrigin
ctLocOrigin CtLoc
loc1)

inertsCanDischarge InertCans
_ Ct
_ = Maybe (CtEvidence, SwapFlag)
forall a. Maybe a
Nothing


interactEq :: InertCans -> Ct -> TcS (StopOrContinue Ct)
interactEq :: InertCans -> SimplifierStage
interactEq InertCans
inerts workItem :: Ct
workItem@(CEqCan { cc_lhs :: Ct -> CanEqLHS
cc_lhs = CanEqLHS
lhs
                                   , cc_rhs :: Ct -> TcPredType
cc_rhs = TcPredType
rhs
                                   , cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev
                                   , cc_eq_rel :: Ct -> EqRel
cc_eq_rel = EqRel
eq_rel })
  | Just (CtEvidence
ev_i, SwapFlag
swapped) <- InertCans -> Ct -> Maybe (CtEvidence, SwapFlag)
inertsCanDischarge InertCans
inerts Ct
workItem
  = do { CtEvidence -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev (EvTerm -> TcS ()) -> EvTerm -> TcS ()
forall a b. (a -> b) -> a -> b
$
         TcCoercion -> EvTerm
evCoercion (SwapFlag -> TcCoercion -> TcCoercion
maybeSymCo SwapFlag
swapped (TcCoercion -> TcCoercion) -> TcCoercion -> TcCoercion
forall a b. (a -> b) -> a -> b
$
                     Role -> Role -> TcCoercion -> TcCoercion
downgradeRole (EqRel -> Role
eqRelRole EqRel
eq_rel)
                                   (CtEvidence -> Role
ctEvRole CtEvidence
ev_i)
                                   ((() :: Constraint) => CtEvidence -> TcCoercion
CtEvidence -> TcCoercion
ctEvCoercion CtEvidence
ev_i))

       ; CtEvidence -> String -> TcS (StopOrContinue Ct)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Solved from inert" }

  | EqRel
ReprEq <- EqRel
eq_rel   -- See Note [Do not unify representational equalities]
  = do { String -> SDoc -> TcS ()
traceTcS String
"Not unifying representational equality" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
workItem)
       ; SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
workItem }

  | Bool
otherwise
  = case CanEqLHS
lhs of
       TyVarLHS TyVar
tv -> Ct -> CtEvidence -> TyVar -> TcPredType -> TcS (StopOrContinue Ct)
tryToSolveByUnification Ct
workItem CtEvidence
ev TyVar
tv TcPredType
rhs

       TyFamLHS TyCon
tc [TcPredType]
args -> do { CtEvidence
-> InertCans -> TyCon -> [TcPredType] -> TcPredType -> TcS ()
improveLocalFunEqs CtEvidence
ev InertCans
inerts TyCon
tc [TcPredType]
args TcPredType
rhs
                              ; SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
workItem }

interactEq InertCans
_ Ct
wi = String -> SDoc -> TcS (StopOrContinue Ct)
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"interactEq" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
wi)

----------------------
-- We have a meta-tyvar on the left, and metaTyVarUpdateOK has said "yes"
-- So try to solve by unifying.
-- Three reasons why not:
--    Skolem escape
--    Given equalities (GADTs)
--    Unifying a TyVarTv with a non-tyvar type
tryToSolveByUnification :: Ct -> CtEvidence
                        -> TcTyVar   -- LHS tyvar
                        -> TcType    -- RHS
                        -> TcS (StopOrContinue Ct)
tryToSolveByUnification :: Ct -> CtEvidence -> TyVar -> TcPredType -> TcS (StopOrContinue Ct)
tryToSolveByUnification Ct
work_item CtEvidence
ev TyVar
tv TcPredType
rhs
  = do { TouchabilityTestResult
is_touchable <- CtFlavour -> TyVar -> TcPredType -> TcS TouchabilityTestResult
touchabilityTest (CtEvidence -> CtFlavour
ctEvFlavour CtEvidence
ev) TyVar
tv TcPredType
rhs
       ; String -> SDoc -> TcS ()
traceTcS String
"tryToSolveByUnification" ([SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tv SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Char -> SDoc
forall doc. IsLine doc => Char -> doc
char Char
'~' SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcPredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcPredType
rhs
                                                  , TouchabilityTestResult -> SDoc
forall a. Outputable a => a -> SDoc
ppr TouchabilityTestResult
is_touchable ])

       ; case TouchabilityTestResult
is_touchable of
           TouchabilityTestResult
Untouchable -> SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
work_item
           -- For the latter two cases see Note [Solve by unification]
           TouchabilityTestResult
TouchableSameLevel -> CtEvidence -> TyVar -> TcPredType -> TcS (StopOrContinue Ct)
solveByUnification CtEvidence
ev TyVar
tv TcPredType
rhs
           TouchableOuterLevel [TyVar]
free_metas TcLevel
tv_lvl
             -> do { TcM () -> TcS ()
forall a. TcM a -> TcS a
wrapTcS (TcM () -> TcS ()) -> TcM () -> TcS ()
forall a b. (a -> b) -> a -> b
$ (TyVar -> IOEnv (Env TcGblEnv TcLclEnv) Bool) -> [TyVar] -> TcM ()
forall (t :: * -> *) (m :: * -> *) a b.
(Foldable t, Monad m) =>
(a -> m b) -> t a -> m ()
mapM_ (TcLevel -> TyVar -> IOEnv (Env TcGblEnv TcLclEnv) Bool
promoteMetaTyVarTo TcLevel
tv_lvl) [TyVar]
free_metas
                   ; TcLevel -> TcS ()
setUnificationFlag TcLevel
tv_lvl
                   ; CtEvidence -> TyVar -> TcPredType -> TcS (StopOrContinue Ct)
solveByUnification CtEvidence
ev TyVar
tv TcPredType
rhs } }

solveByUnification :: CtEvidence -> TcTyVar -> Xi -> TcS (StopOrContinue Ct)
-- Solve with the identity coercion
-- Precondition: kind(xi) equals kind(tv)
-- Precondition: CtEvidence is Wanted
-- Precondition: CtEvidence is nominal
-- Returns: workItem where
--        workItem = the new Given constraint
--
-- NB: No need for an occurs check here, because solveByUnification always
--     arises from a CEqCan, a *canonical* constraint.  Its invariant (TyEq:OC)
--     says that in (a ~ xi), the type variable a does not appear in xi.
--     See GHC.Tc.Types.Constraint.Ct invariants.
--
-- Post: tv is unified (by side effect) with xi;
--       we often write tv := xi
solveByUnification :: CtEvidence -> TyVar -> TcPredType -> TcS (StopOrContinue Ct)
solveByUnification CtEvidence
wd TyVar
tv TcPredType
xi
  = do { let tv_ty :: TcPredType
tv_ty = TyVar -> TcPredType
mkTyVarTy TyVar
tv
       ; String -> SDoc -> TcS ()
traceTcS String
"Sneaky unification:" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
                       [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Unifies:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TyVar -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyVar
tv SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> String -> SDoc
forall doc. IsLine doc => String -> doc
text String
":=" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcPredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcPredType
xi,
                             String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Coercion:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcPredType -> TcPredType -> SDoc
pprEq TcPredType
tv_ty TcPredType
xi,
                             String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Left Kind is:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcPredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr ((() :: Constraint) => TcPredType -> TcPredType
TcPredType -> TcPredType
typeKind TcPredType
tv_ty),
                             String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Right Kind is:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcPredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr ((() :: Constraint) => TcPredType -> TcPredType
TcPredType -> TcPredType
typeKind TcPredType
xi) ]
       ; TyVar -> TcPredType -> TcS ()
unifyTyVar TyVar
tv TcPredType
xi
       ; CtEvidence -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
wd (TcCoercion -> EvTerm
evCoercion (TcPredType -> TcCoercion
mkNomReflCo TcPredType
xi))
       ; ScDepth
n_kicked <- TyVar -> TcS ScDepth
kickOutAfterUnification TyVar
tv
       ; StopOrContinue Ct -> TcS (StopOrContinue Ct)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (CtEvidence -> SDoc -> StopOrContinue Ct
forall a. CtEvidence -> SDoc -> StopOrContinue a
Stop CtEvidence
wd (String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Solved by unification" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> ScDepth -> SDoc
pprKicked ScDepth
n_kicked)) }

{- Note [Avoid double unifications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The spontaneous solver has to return a given which mentions the unified unification
variable *on the left* of the equality. Here is what happens if not:
  Original wanted:  (a ~ alpha),  (alpha ~ Int)
We spontaneously solve the first wanted, without changing the order!
      given : a ~ alpha      [having unified alpha := a]
Now the second wanted comes along, but it cannot rewrite the given, so we simply continue.
At the end we spontaneously solve that guy, *reunifying*  [alpha := Int]

We avoid this problem by orienting the resulting given so that the unification
variable is on the left (note that alternatively we could attempt to
enforce this at canonicalization).

See also Note [No touchables as FunEq RHS] in GHC.Tc.Solver.Monad; avoiding
double unifications is the main reason we disallow touchable
unification variables as RHS of type family equations: F xis ~ alpha.

Note [Do not unify representational equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider   [W] alpha ~R# b
where alpha is touchable. Should we unify alpha := b?

Certainly not!  Unifying forces alpha and be to be the same; but they
only need to be representationally equal types.

For example, we might have another constraint [W] alpha ~# N b
where
  newtype N b = MkN b
and we want to get alpha := N b.

See also #15144, which was caused by unifying a representational
equality.

Note [Solve by unification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we solve
   alpha[n] ~ ty
by unification, there are two cases to consider

* TouchableSameLevel: if the ambient level is 'n', then
  we can simply update alpha := ty, and do nothing else

* TouchableOuterLevel free_metas n: if the ambient level is greater than
  'n' (the level of alpha), in addition to setting alpha := ty we must
  do two other things:

  1. Promote all the free meta-vars of 'ty' to level n.  After all,
     alpha[n] is at level n, and so if we set, say,
          alpha[n] := Maybe beta[m],
     we must ensure that when unifying beta we do skolem-escape checks
     etc relevant to level n.  Simple way to do that: promote beta to
     level n.

  2. Set the Unification Level Flag to record that a level-n unification has
     taken place. See Note [The Unification Level Flag] in GHC.Tc.Solver.Monad

NB: TouchableSameLevel is just an optimisation for TouchableOuterLevel. Promotion
would be a no-op, and setting the unification flag unnecessarily would just
make the solver iterate more often.  (We don't need to iterate when unifying
at the ambient level because of the kick-out mechanism.)


************************************************************************
*                                                                      *
*          Functional dependencies, instantiation of equations
*                                                                      *
************************************************************************

When we spot an equality arising from a functional dependency,
we now use that equality (a "wanted") to rewrite the work-item
constraint right away.  This avoids two dangers

 Danger 1: If we send the original constraint on down the pipeline
           it may react with an instance declaration, and in delicate
           situations (when a Given overlaps with an instance) that
           may produce new insoluble goals: see #4952

 Danger 2: If we don't rewrite the constraint, it may re-react
           with the same thing later, and produce the same equality
           again --> termination worries.

To achieve this required some refactoring of GHC.Tc.Instance.FunDeps (nicer
now!).

Note [FunDep and implicit parameter reactions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Currently, our story of interacting two dictionaries (or a dictionary
and top-level instances) for functional dependencies, and implicit
parameters, is that we simply produce new Wanted equalities.  So for example

        class D a b | a -> b where ...
    Inert:
        d1 :g D Int Bool
    WorkItem:
        d2 :w D Int alpha

    We generate the extra work item
        cv :w alpha ~ Bool
    where 'cv' is currently unused.  However, this new item can perhaps be
    spontaneously solved to become given and react with d2,
    discharging it in favour of a new constraint d2' thus:
        d2' :w D Int Bool
        d2 := d2' |> D Int cv
    Now d2' can be discharged from d1

We could be more aggressive and try to *immediately* solve the dictionary
using those extra equalities.

If that were the case with the same inert set and work item we might discard
d2 directly:

        cv :w alpha ~ Bool
        d2 := d1 |> D Int cv

But in general it's a bit painful to figure out the necessary coercion,
so we just take the first approach. Here is a better example. Consider:
    class C a b c | a -> b
And:
     [Given]  d1 : C T Int Char
     [Wanted] d2 : C T beta Int
In this case, it's *not even possible* to solve the wanted immediately.
So we should simply output the functional dependency and add this guy
[but NOT its superclasses] back in the worklist. Even worse:
     [Given] d1 : C T Int beta
     [Wanted] d2: C T beta Int
Then it is solvable, but its very hard to detect this on the spot.

It's exactly the same with implicit parameters, except that the
"aggressive" approach would be much easier to implement.

Note [Fundeps with instances, and equality orientation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This Note describes a delicate interaction that constrains the orientation of
equalities. This one is about fundeps, but the /exact/ same thing arises for
type-family injectivity constraints: see Note [Improvement orientation].

doTopFundepImprovement compares the constraint with all the instance
declarations, to see if we can produce any equalities. E.g
   class C2 a b | a -> b
   instance C Int Bool
Then the constraint (C Int ty) generates the equality [W] ty ~ Bool.

There is a nasty corner in #19415 which led to the typechecker looping:
   class C s t b | s -> t
   instance ... => C (T kx x) (T ky y) Int
   T :: forall k. k -> Type

   work_item: dwrk :: C (T @ka (a::ka)) (T @kb0 (b0::kb0)) Char
      where kb0, b0 are unification vars

   ==> {doTopFundepImprovement: compare work_item with instance,
        generate /fresh/ unification variables kfresh0, yfresh0,
        emit a new Wanted, and add dwrk to inert set}

   Suppose we emit this new Wanted from the fundep:
       [W] T kb0 (b0::kb0) ~ T kfresh0 (yfresh0::kfresh0)

   ==> {solve that equality kb0 := kfresh0, b0 := yfresh0}
   Now kick out dwrk, since it mentions kb0
   But now we are back to the start!  Loop!

NB1: This example relies on an instance that does not satisfy the
     coverage condition (although it may satisfy the weak coverage
     condition), and hence whose fundeps generate fresh unification
     variables.  Not satisfying the coverage condition is known to
     lead to termination trouble, but in this case it's plain silly.

NB2: In this example, the third parameter to C ensures that the
     instance doesn't actually match the Wanted, so we can't use it to
     solve the Wanted

We solve the problem by (#21703):

    carefully orienting the new Wanted so that all the
    freshly-generated unification variables are on the LHS.

    Thus we emit
       [W] T kfresh0 (yfresh0::kfresh0) ~ T kb0 (b0::kb0)
    and /NOT/
       [W] T kb0 (b0::kb0) ~ T kfresh0 (yfresh0::kfresh0)

Now we'll unify kfresh0:=kb0, yfresh0:=b0, and all is well.  The general idea
is that we want to preferentially eliminate those freshly-generated
unification variables, rather than unifying older variables, which causes
kick-out etc.

Keeping younger variables on the left also gives very minor improvement in
the compiler performance by having less kick-outs and allocations (-0.1% on
average).  Indeed Historical Note [Eliminate younger unification variables]
in GHC.Tc.Utils.Unify describes an earlier attempt to do so systematically,
apparently now in abeyance.

But this is is a delicate solution. We must take care to /preserve/
orientation during solving. Wrinkles:

(W1) We start with
       [W] T kfresh0 (yfresh0::kfresh0) ~ T kb0 (b0::kb0)
     Decompose to
       [W] kfresh0 ~ kb0
       [W] (yfresh0::kfresh0) ~ (b0::kb0)
     Preserve orientiation when decomposing!!

(W2) Suppose we happen to tackle the second Wanted from (W1)
     first. Then in canEqCanLHSHetero we emit a /kind/ equality, as
     well as a now-homogeneous type equality
       [W] kco : kfresh0 ~ kb0
       [W] (yfresh0::kfresh0) ~ (b0::kb0) |> (sym kco)
     Preserve orientation in canEqCanLHSHetero!!  (Failing to
     preserve orientation here was the immediate cause of #21703.)

(W3) There is a potential interaction with the swapping done by
     GHC.Tc.Utils.Unify.swapOverTyVars.  We think it's fine, but it's
     a slight worry.  See especially Note [TyVar/TyVar orientation] in
     that module.

The trouble is that "preserving orientation" is a rather global invariant,
and sometimes we definitely do want to swap (e.g. Int ~ alpha), so we don't
even have a precise statement of what the invariant is.  The advantage
of the preserve-orientation plan is that it is extremely cheap to implement,
and apparently works beautifully.

--- Alternative plan (1) ---
Rather than have an ill-defined invariant, another possiblity is to
elminate those fresh unification variables at birth, when generating
the new fundep-inspired equalities.

The key idea is to call `instFlexiX` in `emitFunDepWanteds` on only those
type variables that are guaranteed to give us some progress. This means we
have to locally (without calling emitWanteds) identify the type variables
that do not give us any progress.  In the above example, we _know_ that
emitting the two wanteds `kco` and `co` is fruitless.

  Q: How do we identify such no-ops?

  1. Generate a matching substitution from LHS to RHS
        ɸ = [kb0 :-> k0, b0 :->  y0]
  2. Call `instFlexiX` on only those type variables that do not appear in the domain of ɸ
        ɸ' = instFlexiX ɸ (tvs - domain ɸ)
  3. Apply ɸ' on LHS and then call emitWanteds
        unifyWanteds ... (subst ɸ' LHS) RHS

Why will this work?  The matching substitution ɸ will be a best effort
substitution that gives us all the easy solutions. It can be generated with
modified version of `Core/Unify.unify_tys` where we run it in a matching mode
and never generate `SurelyApart` and always return a `MaybeApart Subst`
instead.

The same alternative plan would work for type-family injectivity constraints:
see Note [Improvement orientation].
--- End of Alternative plan (1) ---

--- Alternative plan (2) ---
We could have a new flavour of TcTyVar (like `TauTv`, `TyVarTv` etc; see GHC.Tc.Utils.TcType.MetaInfo)
for the fresh unification variables introduced by functional dependencies.  Say `FunDepTv`.  Then in
GHC.Tc.Utils.Unify.swapOverTyVars we could arrange to keep a `FunDepTv` on the left if possible.
Looks possible, but it's one more complication.
--- End of Alternative plan (2) ---


--- Historical note: Failed Alternative Plan (3) ---
Previously we used a flag `cc_fundeps` in `CDictCan`. It would flip to False
once we used a fun dep to hint the solver to break and to stop emitting more
wanteds.  This solution was not complete, and caused a failures while trying
to solve for transitive functional dependencies (test case: T21703)
-- End of Historical note: Failed Alternative Plan (3) --

Note [Weird fundeps]
~~~~~~~~~~~~~~~~~~~~
Consider   class Het a b | a -> b where
              het :: m (f c) -> a -> m b

           class GHet (a :: * -> *) (b :: * -> *) | a -> b
           instance            GHet (K a) (K [a])
           instance Het a b => GHet (K a) (K b)

The two instances don't actually conflict on their fundeps,
although it's pretty strange.  So they are both accepted. Now
try   [W] GHet (K Int) (K Bool)
This triggers fundeps from both instance decls;
      [W] K Bool ~ K [a]
      [W] K Bool ~ K beta
And there's a risk of complaining about Bool ~ [a].  But in fact
the Wanted matches the second instance, so we never get as far
as the fundeps.

#7875 is a case in point.
-}

doTopFundepImprovement :: Ct -> TcS ()
-- Try to functional-dependency improvement between the constraint
-- and the top-level instance declarations
-- See Note [Fundeps with instances, and equality orientation]
-- See also Note [Weird fundeps]
doTopFundepImprovement :: Ct -> TcS ()
doTopFundepImprovement work_item :: Ct
work_item@(CDictCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev, cc_class :: Ct -> Class
cc_class = Class
cls
                                           , cc_tyargs :: Ct -> [TcPredType]
cc_tyargs = [TcPredType]
xis })
  = do { String -> SDoc -> TcS ()
traceTcS String
"try_fundeps" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
work_item)
       ; InstEnvs
instEnvs <- TcS InstEnvs
getInstEnvs
       ; let fundep_eqns :: [FunDepEqn (CtLoc, RewriterSet)]
fundep_eqns = InstEnvs
-> (TcPredType -> SrcSpan -> (CtLoc, RewriterSet))
-> Class
-> [TcPredType]
-> [FunDepEqn (CtLoc, RewriterSet)]
forall loc.
InstEnvs
-> (TcPredType -> SrcSpan -> loc)
-> Class
-> [TcPredType]
-> [FunDepEqn loc]
improveFromInstEnv InstEnvs
instEnvs TcPredType -> SrcSpan -> (CtLoc, RewriterSet)
mk_ct_loc Class
cls [TcPredType]
xis
       ; RewriterSet -> [FunDepEqn (CtLoc, RewriterSet)] -> TcS ()
emitFunDepWanteds (CtEvidence -> RewriterSet
ctEvRewriters CtEvidence
ev) [FunDepEqn (CtLoc, RewriterSet)]
fundep_eqns }
  where
     dict_pred :: TcPredType
dict_pred   = Class -> [TcPredType] -> TcPredType
mkClassPred Class
cls [TcPredType]
xis
     dict_loc :: CtLoc
dict_loc    = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev
     dict_origin :: CtOrigin
dict_origin = CtLoc -> CtOrigin
ctLocOrigin CtLoc
dict_loc

     mk_ct_loc :: PredType   -- From instance decl
               -> SrcSpan    -- also from instance deol
               -> (CtLoc, RewriterSet)
     mk_ct_loc :: TcPredType -> SrcSpan -> (CtLoc, RewriterSet)
mk_ct_loc TcPredType
inst_pred SrcSpan
inst_loc
       = ( CtLoc
dict_loc { ctl_origin = FunDepOrigin2 dict_pred dict_origin
                                                 inst_pred inst_loc }
         , RewriterSet
emptyRewriterSet )

doTopFundepImprovement Ct
work_item = String -> SDoc -> TcS ()
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"doTopFundepImprovement" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
work_item)

emitFunDepWanteds :: RewriterSet  -- from the work item
                  -> [FunDepEqn (CtLoc, RewriterSet)] -> TcS ()

emitFunDepWanteds :: RewriterSet -> [FunDepEqn (CtLoc, RewriterSet)] -> TcS ()
emitFunDepWanteds RewriterSet
_ [] = () -> TcS ()
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return () -- common case noop
-- See Note [FunDep and implicit parameter reactions]

emitFunDepWanteds RewriterSet
work_rewriters [FunDepEqn (CtLoc, RewriterSet)]
fd_eqns
  = (FunDepEqn (CtLoc, RewriterSet) -> TcS ())
-> [FunDepEqn (CtLoc, RewriterSet)] -> TcS ()
forall (t :: * -> *) (m :: * -> *) a b.
(Foldable t, Monad m) =>
(a -> m b) -> t a -> m ()
mapM_ FunDepEqn (CtLoc, RewriterSet) -> TcS ()
do_one_FDEqn [FunDepEqn (CtLoc, RewriterSet)]
fd_eqns
  where
    do_one_FDEqn :: FunDepEqn (CtLoc, RewriterSet) -> TcS ()
do_one_FDEqn (FDEqn { fd_qtvs :: forall loc. FunDepEqn loc -> [TyVar]
fd_qtvs = [TyVar]
tvs, fd_eqs :: forall loc. FunDepEqn loc -> [TypeEqn]
fd_eqs = [TypeEqn]
eqs, fd_loc :: forall loc. FunDepEqn loc -> loc
fd_loc = (CtLoc
loc, RewriterSet
rewriters) })
     | [TyVar] -> Bool
forall a. [a] -> Bool
forall (t :: * -> *) a. Foldable t => t a -> Bool
null [TyVar]
tvs  -- Common shortcut
     = do { String -> SDoc -> TcS ()
traceTcS String
"emitFunDepWanteds 1" (SubGoalDepth -> SDoc
forall a. Outputable a => a -> SDoc
ppr (CtLoc -> SubGoalDepth
ctl_depth CtLoc
loc) SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ [TypeEqn] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TypeEqn]
eqs SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ Bool -> SDoc
forall a. Outputable a => a -> SDoc
ppr (CtLoc -> Bool
isGivenLoc CtLoc
loc))
          ; (TypeEqn -> TcS TcCoercion) -> [TypeEqn] -> TcS ()
forall (t :: * -> *) (m :: * -> *) a b.
(Foldable t, Monad m) =>
(a -> m b) -> t a -> m ()
mapM_ (\(Pair TcPredType
ty1 TcPredType
ty2) -> RewriterSet
-> CtLoc -> Role -> TcPredType -> TcPredType -> TcS TcCoercion
unifyWanted RewriterSet
all_rewriters CtLoc
loc Role
Nominal TcPredType
ty1 TcPredType
ty2)
                  ([TypeEqn] -> [TypeEqn]
forall a. [a] -> [a]
reverse [TypeEqn]
eqs) }
             -- See Note [Reverse order of fundep equations]

     | Bool
otherwise
     = do { String -> SDoc -> TcS ()
traceTcS String
"emitFunDepWanteds 2" (SubGoalDepth -> SDoc
forall a. Outputable a => a -> SDoc
ppr (CtLoc -> SubGoalDepth
ctl_depth CtLoc
loc) SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ [TyVar] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TyVar]
tvs SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ [TypeEqn] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TypeEqn]
eqs)
          ; Subst
subst <- Subst -> [TyVar] -> TcS Subst
instFlexiX Subst
emptySubst [TyVar]
tvs  -- Takes account of kind substitution
          ; (TypeEqn -> TcS TcCoercion) -> [TypeEqn] -> TcS ()
forall (t :: * -> *) (m :: * -> *) a b.
(Foldable t, Monad m) =>
(a -> m b) -> t a -> m ()
mapM_ (CtLoc -> RewriterSet -> Subst -> TypeEqn -> TcS TcCoercion
do_one_eq CtLoc
loc RewriterSet
all_rewriters Subst
subst) ([TypeEqn] -> [TypeEqn]
forall a. [a] -> [a]
reverse [TypeEqn]
eqs) }
               -- See Note [Reverse order of fundep equations]
     where
       all_rewriters :: RewriterSet
all_rewriters = RewriterSet
work_rewriters RewriterSet -> RewriterSet -> RewriterSet
forall a. Semigroup a => a -> a -> a
S.<> RewriterSet
rewriters

    do_one_eq :: CtLoc -> RewriterSet -> Subst -> TypeEqn -> TcS TcCoercion
do_one_eq CtLoc
loc RewriterSet
rewriters Subst
subst (Pair TcPredType
ty1 TcPredType
ty2)
       = RewriterSet
-> CtLoc -> Role -> TcPredType -> TcPredType -> TcS TcCoercion
unifyWanted RewriterSet
rewriters CtLoc
loc Role
Nominal (Subst -> TcPredType -> TcPredType
substTyUnchecked Subst
subst' TcPredType
ty1) TcPredType
ty2
         -- ty2 does not mention fd_qtvs, so no need to subst it.
         -- See GHC.Tc.Instance.Fundeps Note [Improving against instances]
         --     Wrinkle (1)
      where
         subst' :: Subst
subst' = Subst -> VarSet -> Subst
extendSubstInScopeSet Subst
subst (TcPredType -> VarSet
tyCoVarsOfType TcPredType
ty1)
         -- The free vars of ty1 aren't just fd_qtvs: ty1 is the result
         -- of matching with the [W] constraint. So we add its free
         -- vars to InScopeSet, to satisfy substTy's invariants, even
         -- though ty1 will never (currently) be a poytype, so this
         -- InScopeSet will never be looked at.

{-
**********************************************************************
*                                                                    *
                       The top-reaction Stage
*                                                                    *
**********************************************************************
-}

topReactionsStage :: WorkItem -> TcS (StopOrContinue Ct)
-- The work item does not react with the inert set,
-- so try interaction with top-level instances.
topReactionsStage :: SimplifierStage
topReactionsStage Ct
work_item
  = do { String -> SDoc -> TcS ()
traceTcS String
"doTopReact" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
work_item)
       ; case Ct
work_item of

           CDictCan {} ->
             do { InertSet
inerts <- TcS InertSet
getTcSInerts
                ; InertSet -> SimplifierStage
doTopReactDict InertSet
inerts Ct
work_item }

           CEqCan {} ->
             SimplifierStage
doTopReactEq Ct
work_item

           CIrredCan {} ->
             SimplifierStage
doTopReactOther Ct
work_item

           -- Any other work item does not react with any top-level equations
           Ct
_  -> SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
work_item }

--------------------
doTopReactOther :: Ct -> TcS (StopOrContinue Ct)
-- Try local quantified constraints for
--     CEqCan    e.g.  (lhs ~# ty)
-- and CIrredCan e.g.  (c a)
--
-- Why equalities? See GHC.Tc.Solver.Canonical
-- Note [Equality superclasses in quantified constraints]
doTopReactOther :: SimplifierStage
doTopReactOther Ct
work_item
  | CtEvidence -> Bool
isGiven CtEvidence
ev
  = SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
work_item

  | EqPred EqRel
eq_rel TcPredType
t1 TcPredType
t2 <- TcPredType -> Pred
classifyPredType TcPredType
pred
  = Ct -> EqRel -> TcPredType -> TcPredType -> TcS (StopOrContinue Ct)
doTopReactEqPred Ct
work_item EqRel
eq_rel TcPredType
t1 TcPredType
t2

  | Bool
otherwise
  = do { ClsInstResult
res <- TcPredType -> CtLoc -> TcS ClsInstResult
matchLocalInst TcPredType
pred CtLoc
loc
       ; case ClsInstResult
res of
           OneInst {} -> Ct -> ClsInstResult -> TcS (StopOrContinue Ct)
chooseInstance Ct
work_item ClsInstResult
res
           ClsInstResult
_          -> SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
work_item }

  where
    ev :: CtEvidence
ev   = Ct -> CtEvidence
ctEvidence Ct
work_item
    loc :: CtLoc
loc  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev
    pred :: TcPredType
pred = CtEvidence -> TcPredType
ctEvPred CtEvidence
ev

{-********************************************************************
*                                                                    *
          Top-level reaction for equality constraints (CEqCan)
*                                                                    *
********************************************************************-}

doTopReactEqPred :: Ct -> EqRel -> TcType -> TcType -> TcS (StopOrContinue Ct)
doTopReactEqPred :: Ct -> EqRel -> TcPredType -> TcPredType -> TcS (StopOrContinue Ct)
doTopReactEqPred Ct
work_item EqRel
eq_rel TcPredType
t1 TcPredType
t2
  -- See Note [Looking up primitive equalities in quantified constraints]
  | Just (Class
cls, [TcPredType]
tys) <- EqRel -> TcPredType -> TcPredType -> Maybe (Class, [TcPredType])
boxEqPred EqRel
eq_rel TcPredType
t1 TcPredType
t2
  = do { ClsInstResult
res <- TcPredType -> CtLoc -> TcS ClsInstResult
matchLocalInst (Class -> [TcPredType] -> TcPredType
mkClassPred Class
cls [TcPredType]
tys) CtLoc
loc
       ; case ClsInstResult
res of
           OneInst { cir_mk_ev :: ClsInstResult -> [EvExpr] -> EvTerm
cir_mk_ev = [EvExpr] -> EvTerm
mk_ev }
             -> Ct -> ClsInstResult -> TcS (StopOrContinue Ct)
chooseInstance Ct
work_item
                    (ClsInstResult
res { cir_mk_ev = mk_eq_ev cls tys mk_ev })
           ClsInstResult
_ -> SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
work_item }

  | Bool
otherwise
  = SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
work_item
  where
    ev :: CtEvidence
ev   = Ct -> CtEvidence
ctEvidence Ct
work_item
    loc :: CtLoc
loc = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev

    mk_eq_ev :: Class -> [TcPredType] -> (t -> EvTerm) -> t -> EvTerm
mk_eq_ev Class
cls [TcPredType]
tys t -> EvTerm
mk_ev t
evs
      = case (t -> EvTerm
mk_ev t
evs) of
          EvExpr EvExpr
e -> EvExpr -> EvTerm
EvExpr (TyVar -> EvExpr
forall b. TyVar -> Expr b
Var TyVar
sc_id EvExpr -> [TcPredType] -> EvExpr
forall b. Expr b -> [TcPredType] -> Expr b
`mkTyApps` [TcPredType]
tys EvExpr -> EvExpr -> EvExpr
forall b. Expr b -> Expr b -> Expr b
`App` EvExpr
e)
          EvTerm
ev       -> String -> SDoc -> EvTerm
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"mk_eq_ev" (EvTerm -> SDoc
forall a. Outputable a => a -> SDoc
ppr EvTerm
ev)
      where
        [TyVar
sc_id] = Class -> [TyVar]
classSCSelIds Class
cls

{- Note [Looking up primitive equalities in quantified constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For equalities (a ~# b) look up (a ~ b), and then do a superclass
selection. This avoids having to support quantified constraints whose
kind is not Constraint, such as (forall a. F a ~# b)

See
 * Note [Evidence for quantified constraints] in GHC.Core.Predicate
 * Note [Equality superclasses in quantified constraints]
   in GHC.Tc.Solver.Canonical

Note [Reverse order of fundep equations]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this scenario (from dependent/should_fail/T13135_simple):

  type Sig :: Type -> Type
  data Sig a = SigFun a (Sig a)

  type SmartFun :: forall (t :: Type). Sig t -> Type
  type family SmartFun sig = r | r -> sig where
    SmartFun @Type (SigFun @Type a sig) = a -> SmartFun @Type sig

  [W] SmartFun @kappa sigma ~ (Int -> Bool)

The injectivity of SmartFun allows us to produce two new equalities:

  [W] w1 :: Type ~ kappa
  [W] w2 :: SigFun @Type Int beta ~ sigma

for some fresh (beta :: SigType). The second Wanted here is actually
heterogeneous: the LHS has type Sig Type while the RHS has type Sig kappa.
Of course, if we solve the first wanted first, the second becomes homogeneous.

When looking for injectivity-inspired equalities, we work left-to-right,
producing the two equalities in the order written above. However, these
equalities are then passed into unifyWanted, which will fail, adding these
to the work list. However, crucially, the work list operates like a *stack*.
So, because we add w1 and then w2, we process w2 first. This is silly: solving
w1 would unlock w2. So we make sure to add equalities to the work
list in left-to-right order, which requires a few key calls to 'reverse'.

This treatment is also used for class-based functional dependencies, although
we do not have a program yet known to exhibit a loop there. It just seems
like the right thing to do.

When this was originally conceived, it was necessary to avoid a loop in T13135.
That loop is now avoided by continuing with the kind equality (not the type
equality) in canEqCanLHSHetero (see Note [Equalities with incompatible kinds]
in GHC.Tc.Solver.Canonical). However, the idea of working left-to-right still
seems worthwhile, and so the calls to 'reverse' remain.

-}

--------------------
doTopReactEq :: Ct -> TcS (StopOrContinue Ct)
doTopReactEq :: SimplifierStage
doTopReactEq work_item :: Ct
work_item@(CEqCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
old_ev, cc_lhs :: Ct -> CanEqLHS
cc_lhs = TyFamLHS TyCon
fam_tc [TcPredType]
args
                               , cc_rhs :: Ct -> TcPredType
cc_rhs = TcPredType
rhs })
  = do { CtEvidence -> TyCon -> [TcPredType] -> TcPredType -> TcS ()
improveTopFunEqs CtEvidence
old_ev TyCon
fam_tc [TcPredType]
args TcPredType
rhs
       ; SimplifierStage
doTopReactOther Ct
work_item }
doTopReactEq Ct
work_item = SimplifierStage
doTopReactOther Ct
work_item

improveTopFunEqs :: CtEvidence -> TyCon -> [TcType] -> TcType -> TcS ()
-- See Note [FunDep and implicit parameter reactions]
improveTopFunEqs :: CtEvidence -> TyCon -> [TcPredType] -> TcPredType -> TcS ()
improveTopFunEqs CtEvidence
ev TyCon
fam_tc [TcPredType]
args TcPredType
rhs
  | CtEvidence -> Bool
isGiven CtEvidence
ev  -- See Note [No Given/Given fundeps]
  = () -> TcS ()
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ()

  | Bool
otherwise
  = do { (FamInstEnv, FamInstEnv)
fam_envs <- TcS (FamInstEnv, FamInstEnv)
getFamInstEnvs
       ; [TypeEqn]
eqns <- (FamInstEnv, FamInstEnv)
-> TyCon -> [TcPredType] -> TcPredType -> TcS [TypeEqn]
improve_top_fun_eqs (FamInstEnv, FamInstEnv)
fam_envs TyCon
fam_tc [TcPredType]
args TcPredType
rhs
       ; String -> SDoc -> TcS ()
traceTcS String
"improveTopFunEqs" ([SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ TyCon -> SDoc
forall a. Outputable a => a -> SDoc
ppr TyCon
fam_tc SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [TcPredType] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TcPredType]
args SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcPredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcPredType
rhs
                                          , [TypeEqn] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [TypeEqn]
eqns ])
       ; (TypeEqn -> TcS TcCoercion) -> [TypeEqn] -> TcS ()
forall (t :: * -> *) (m :: * -> *) a b.
(Foldable t, Monad m) =>
(a -> m b) -> t a -> m ()
mapM_ (\(Pair TcPredType
ty1 TcPredType
ty2) -> RewriterSet
-> CtLoc -> Role -> TcPredType -> TcPredType -> TcS TcCoercion
unifyWanted RewriterSet
rewriters CtLoc
loc Role
Nominal TcPredType
ty1 TcPredType
ty2)
               ([TypeEqn] -> [TypeEqn]
forall a. [a] -> [a]
reverse [TypeEqn]
eqns) }
         -- Missing that `reverse` causes T13135 and T13135_simple to loop.
         -- See Note [Reverse order of fundep equations]
  where
    loc :: CtLoc
loc = CtLoc -> CtLoc
bumpCtLocDepth (CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev)
        -- ToDo: this location is wrong; it should be FunDepOrigin2
        -- See #14778
    rewriters :: RewriterSet
rewriters = CtEvidence -> RewriterSet
ctEvRewriters CtEvidence
ev

improve_top_fun_eqs :: FamInstEnvs
                    -> TyCon -> [TcType] -> TcType
                    -> TcS [TypeEqn]
improve_top_fun_eqs :: (FamInstEnv, FamInstEnv)
-> TyCon -> [TcPredType] -> TcPredType -> TcS [TypeEqn]
improve_top_fun_eqs (FamInstEnv, FamInstEnv)
fam_envs TyCon
fam_tc [TcPredType]
args TcPredType
rhs_ty
  | Just BuiltInSynFamily
ops <- TyCon -> Maybe BuiltInSynFamily
isBuiltInSynFamTyCon_maybe TyCon
fam_tc
  = [TypeEqn] -> TcS [TypeEqn]
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (BuiltInSynFamily -> [TcPredType] -> TcPredType -> [TypeEqn]
sfInteractTop BuiltInSynFamily
ops [TcPredType]
args TcPredType
rhs_ty)

  -- see Note [Type inference for type families with injectivity]
  | TyCon -> Bool
isOpenTypeFamilyTyCon TyCon
fam_tc
  , Injective [Bool]
injective_args <- TyCon -> Injectivity
tyConInjectivityInfo TyCon
fam_tc
  , let fam_insts :: [FamInst]
fam_insts = (FamInstEnv, FamInstEnv) -> TyCon -> [FamInst]
lookupFamInstEnvByTyCon (FamInstEnv, FamInstEnv)
fam_envs TyCon
fam_tc
  = -- it is possible to have several compatible equations in an open type
    -- family but we only want to derive equalities from one such equation.
    do { let improvs :: [([TcPredType], Subst, [TyVar], Maybe CoAxBranch)]
improvs = [FamInst]
-> (FamInst -> [TyVar])
-> (FamInst -> [TcPredType])
-> (FamInst -> TcPredType)
-> (FamInst -> Maybe CoAxBranch)
-> [([TcPredType], Subst, [TyVar], Maybe CoAxBranch)]
forall a.
[a]
-> (a -> [TyVar])
-> (a -> [TcPredType])
-> (a -> TcPredType)
-> (a -> Maybe CoAxBranch)
-> [([TcPredType], Subst, [TyVar], Maybe CoAxBranch)]
buildImprovementData [FamInst]
fam_insts
                           FamInst -> [TyVar]
fi_tvs FamInst -> [TcPredType]
fi_tys FamInst -> TcPredType
fi_rhs (Maybe CoAxBranch -> FamInst -> Maybe CoAxBranch
forall a b. a -> b -> a
const Maybe CoAxBranch
forall a. Maybe a
Nothing)

       ; String -> SDoc -> TcS ()
traceTcS String
"improve_top_fun_eqs2" ([([TcPredType], Subst, [TyVar], Maybe CoAxBranch)] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [([TcPredType], Subst, [TyVar], Maybe CoAxBranch)]
improvs)
       ; (([TcPredType], Subst, [TyVar], Maybe CoAxBranch) -> TcS [TypeEqn])
-> [([TcPredType], Subst, [TyVar], Maybe CoAxBranch)]
-> TcS [TypeEqn]
forall (m :: * -> *) (f :: * -> *) a b.
(Monad m, Traversable f) =>
(a -> m [b]) -> f a -> m [b]
concatMapM ([Bool]
-> ([TcPredType], Subst, [TyVar], Maybe CoAxBranch)
-> TcS [TypeEqn]
injImproveEqns [Bool]
injective_args) ([([TcPredType], Subst, [TyVar], Maybe CoAxBranch)]
 -> TcS [TypeEqn])
-> [([TcPredType], Subst, [TyVar], Maybe CoAxBranch)]
-> TcS [TypeEqn]
forall a b. (a -> b) -> a -> b
$
         ScDepth
-> [([TcPredType], Subst, [TyVar], Maybe CoAxBranch)]
-> [([TcPredType], Subst, [TyVar], Maybe CoAxBranch)]
forall a. ScDepth -> [a] -> [a]
take ScDepth
1 [([TcPredType], Subst, [TyVar], Maybe CoAxBranch)]
improvs }

  | Just CoAxiom Branched
ax <- TyCon -> Maybe (CoAxiom Branched)
isClosedSynFamilyTyConWithAxiom_maybe TyCon
fam_tc
  , Injective [Bool]
injective_args <- TyCon -> Injectivity
tyConInjectivityInfo TyCon
fam_tc
  = (([TcPredType], Subst, [TyVar], Maybe CoAxBranch) -> TcS [TypeEqn])
-> [([TcPredType], Subst, [TyVar], Maybe CoAxBranch)]
-> TcS [TypeEqn]
forall (m :: * -> *) (f :: * -> *) a b.
(Monad m, Traversable f) =>
(a -> m [b]) -> f a -> m [b]
concatMapM ([Bool]
-> ([TcPredType], Subst, [TyVar], Maybe CoAxBranch)
-> TcS [TypeEqn]
injImproveEqns [Bool]
injective_args) ([([TcPredType], Subst, [TyVar], Maybe CoAxBranch)]
 -> TcS [TypeEqn])
-> [([TcPredType], Subst, [TyVar], Maybe CoAxBranch)]
-> TcS [TypeEqn]
forall a b. (a -> b) -> a -> b
$
    [CoAxBranch]
-> (CoAxBranch -> [TyVar])
-> (CoAxBranch -> [TcPredType])
-> (CoAxBranch -> TcPredType)
-> (CoAxBranch -> Maybe CoAxBranch)
-> [([TcPredType], Subst, [TyVar], Maybe CoAxBranch)]
forall a.
[a]
-> (a -> [TyVar])
-> (a -> [TcPredType])
-> (a -> TcPredType)
-> (a -> Maybe CoAxBranch)
-> [([TcPredType], Subst, [TyVar], Maybe CoAxBranch)]
buildImprovementData (Branches Branched -> [CoAxBranch]
forall (br :: BranchFlag). Branches br -> [CoAxBranch]
fromBranches (CoAxiom Branched -> Branches Branched
forall (br :: BranchFlag). CoAxiom br -> Branches br
co_ax_branches CoAxiom Branched
ax))
                         CoAxBranch -> [TyVar]
cab_tvs CoAxBranch -> [TcPredType]
cab_lhs CoAxBranch -> TcPredType
cab_rhs CoAxBranch -> Maybe CoAxBranch
forall a. a -> Maybe a
Just

  | Bool
otherwise
  = [TypeEqn] -> TcS [TypeEqn]
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return []

  where
      in_scope :: InScopeSet
in_scope = VarSet -> InScopeSet
mkInScopeSet (TcPredType -> VarSet
tyCoVarsOfType TcPredType
rhs_ty)

      buildImprovementData
          :: [a]                     -- axioms for a TF (FamInst or CoAxBranch)
          -> (a -> [TyVar])          -- get bound tyvars of an axiom
          -> (a -> [Type])           -- get LHS of an axiom
          -> (a -> Type)             -- get RHS of an axiom
          -> (a -> Maybe CoAxBranch) -- Just => apartness check required
          -> [( [Type], Subst, [TyVar], Maybe CoAxBranch )]
             -- Result:
             -- ( [arguments of a matching axiom]
             -- , RHS-unifying substitution
             -- , axiom variables without substitution
             -- , Maybe matching axiom [Nothing - open TF, Just - closed TF ] )
      buildImprovementData :: forall a.
[a]
-> (a -> [TyVar])
-> (a -> [TcPredType])
-> (a -> TcPredType)
-> (a -> Maybe CoAxBranch)
-> [([TcPredType], Subst, [TyVar], Maybe CoAxBranch)]
buildImprovementData [a]
axioms a -> [TyVar]
axiomTVs a -> [TcPredType]
axiomLHS a -> TcPredType
axiomRHS a -> Maybe CoAxBranch
wrap =
          [ ([TcPredType]
ax_args, Subst
subst, [TyVar]
unsubstTvs, a -> Maybe CoAxBranch
wrap a
axiom)
          | a
axiom <- [a]
axioms
          , let ax_args :: [TcPredType]
ax_args = a -> [TcPredType]
axiomLHS a
axiom
                ax_rhs :: TcPredType
ax_rhs  = a -> TcPredType
axiomRHS a
axiom
                ax_tvs :: [TyVar]
ax_tvs  = a -> [TyVar]
axiomTVs a
axiom
                in_scope1 :: InScopeSet
in_scope1 = InScopeSet
in_scope InScopeSet -> [TyVar] -> InScopeSet
`extendInScopeSetList` [TyVar]
ax_tvs
          , Just Subst
subst <- [Bool -> InScopeSet -> TcPredType -> TcPredType -> Maybe Subst
tcUnifyTyWithTFs Bool
False InScopeSet
in_scope1 TcPredType
ax_rhs TcPredType
rhs_ty]
          , let notInSubst :: TyVar -> Bool
notInSubst TyVar
tv = Bool -> Bool
not (TyVar
tv TyVar -> VarEnv TcPredType -> Bool
forall a. TyVar -> VarEnv a -> Bool
`elemVarEnv` Subst -> VarEnv TcPredType
getTvSubstEnv Subst
subst)
                unsubstTvs :: [TyVar]
unsubstTvs    = (TyVar -> Bool) -> [TyVar] -> [TyVar]
forall a. (a -> Bool) -> [a] -> [a]
filter (TyVar -> Bool
notInSubst (TyVar -> Bool) -> (TyVar -> Bool) -> TyVar -> Bool
forall (f :: * -> *). Applicative f => f Bool -> f Bool -> f Bool
<&&> TyVar -> Bool
isTyVar) [TyVar]
ax_tvs ]
                   -- The order of unsubstTvs is important; it must be
                   -- in telescope order e.g. (k:*) (a:k)

      injImproveEqns :: [Bool]
                     -> ([Type], Subst, [TyCoVar], Maybe CoAxBranch)
                     -> TcS [TypeEqn]
      injImproveEqns :: [Bool]
-> ([TcPredType], Subst, [TyVar], Maybe CoAxBranch)
-> TcS [TypeEqn]
injImproveEqns [Bool]
inj_args ([TcPredType]
ax_args, Subst
subst, [TyVar]
unsubstTvs, Maybe CoAxBranch
cabr)
        = do { Subst
subst <- Subst -> [TyVar] -> TcS Subst
instFlexiX Subst
subst [TyVar]
unsubstTvs
                  -- If the current substitution bind [k -> *], and
                  -- one of the un-substituted tyvars is (a::k), we'd better
                  -- be sure to apply the current substitution to a's kind.
                  -- Hence instFlexiX.   #13135 was an example.

             ; [TypeEqn] -> TcS [TypeEqn]
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return [ TcPredType -> TcPredType -> TypeEqn
forall a. a -> a -> Pair a
Pair ((() :: Constraint) => Subst -> TcPredType -> TcPredType
Subst -> TcPredType -> TcPredType
substTy Subst
subst TcPredType
ax_arg) TcPredType
arg
                        -- NB: the ax_arg part is on the left
                        -- see Note [Improvement orientation]
                      | case Maybe CoAxBranch
cabr of
                          Just CoAxBranch
cabr' -> [TcPredType] -> CoAxBranch -> Bool
apartnessCheck ((() :: Constraint) => Subst -> [TcPredType] -> [TcPredType]
Subst -> [TcPredType] -> [TcPredType]
substTys Subst
subst [TcPredType]
ax_args) CoAxBranch
cabr'
                          Maybe CoAxBranch
_          -> Bool
True
                      , (TcPredType
ax_arg, TcPredType
arg, Bool
True) <- [TcPredType]
-> [TcPredType] -> [Bool] -> [(TcPredType, TcPredType, Bool)]
forall a b c. [a] -> [b] -> [c] -> [(a, b, c)]
zip3 [TcPredType]
ax_args [TcPredType]
args [Bool]
inj_args ] }

{-
Note [MATCHING-SYNONYMS]
~~~~~~~~~~~~~~~~~~~~~~~~
When trying to match a dictionary (D tau) to a top-level instance, or a
type family equation (F taus_1 ~ tau_2) to a top-level family instance,
we do *not* need to expand type synonyms because the matcher will do that for us.

Note [Improvement orientation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
See also Note [Fundeps with instances, and equality orientation], which describes
the Exact Same Prolem, with the same solution, but for functional dependencies.

A very delicate point is the orientation of equalities
arising from injectivity improvement (#12522).  Suppose we have
  type family F x = t | t -> x
  type instance F (a, Int) = (Int, G a)
where G is injective; and wanted constraints

  [W] TF (alpha, beta) ~ fuv
  [W] fuv ~ (Int, <some type>)

The injectivity will give rise to constraints

  [W] gamma1 ~ alpha
  [W] Int ~ beta

The fresh unification variable gamma1 comes from the fact that we
can only do "partial improvement" here; see Section 5.2 of
"Injective type families for Haskell" (HS'15).

Now, it's very important to orient the equations this way round,
so that the fresh unification variable will be eliminated in
favour of alpha.  If we instead had
   [W] alpha ~ gamma1
then we would unify alpha := gamma1; and kick out the wanted
constraint.  But when we grough it back in, it'd look like
   [W] TF (gamma1, beta) ~ fuv
and exactly the same thing would happen again!  Infinite loop.

This all seems fragile, and it might seem more robust to avoid
introducing gamma1 in the first place, in the case where the
actual argument (alpha, beta) partly matches the improvement
template.  But that's a bit tricky, esp when we remember that the
kinds much match too; so it's easier to let the normal machinery
handle it.  Instead we are careful to orient the new
equality with the template on the left.  Delicate, but it works.

-}

{- *******************************************************************
*                                                                    *
         Top-level reaction for class constraints (CDictCan)
*                                                                    *
**********************************************************************-}

doTopReactDict :: InertSet -> Ct -> TcS (StopOrContinue Ct)
-- Try to use type-class instance declarations to simplify the constraint
doTopReactDict :: InertSet -> SimplifierStage
doTopReactDict InertSet
inerts work_item :: Ct
work_item@(CDictCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev, cc_class :: Ct -> Class
cc_class = Class
cls
                                          , cc_tyargs :: Ct -> [TcPredType]
cc_tyargs = [TcPredType]
xis })
  | CtEvidence -> Bool
isGiven CtEvidence
ev   -- Never use instances for Given constraints
  = SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
work_item
     -- See Note [No Given/Given fundeps]

  | Just CtEvidence
solved_ev <- InertSet -> CtLoc -> Class -> [TcPredType] -> Maybe CtEvidence
lookupSolvedDict InertSet
inerts CtLoc
dict_loc Class
cls [TcPredType]
xis   -- Cached
  = do { CtEvidence -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev (CtEvidence -> EvTerm
ctEvTerm CtEvidence
solved_ev)
       ; CtEvidence -> String -> TcS (StopOrContinue Ct)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Dict/Top (cached)" }

  | Bool
otherwise  -- Wanted, but not cached
   = do { DynFlags
dflags <- TcS DynFlags
forall (m :: * -> *). HasDynFlags m => m DynFlags
getDynFlags
        ; ClsInstResult
lkup_res <- DynFlags
-> InertSet -> Class -> [TcPredType] -> CtLoc -> TcS ClsInstResult
matchClassInst DynFlags
dflags InertSet
inerts Class
cls [TcPredType]
xis CtLoc
dict_loc
        ; case ClsInstResult
lkup_res of
               OneInst { cir_what :: ClsInstResult -> InstanceWhat
cir_what = InstanceWhat
what }
                  -> do { InstanceWhat -> Ct -> TcS ()
insertSafeOverlapFailureTcS InstanceWhat
what Ct
work_item
                        ; InstanceWhat -> CtEvidence -> Class -> [TcPredType] -> TcS ()
addSolvedDict InstanceWhat
what CtEvidence
ev Class
cls [TcPredType]
xis
                        ; Ct -> ClsInstResult -> TcS (StopOrContinue Ct)
chooseInstance Ct
work_item ClsInstResult
lkup_res }
               ClsInstResult
_  -> -- NoInstance or NotSure
                     -- We didn't solve it; so try functional dependencies with
                     -- the instance environment
                     do { Ct -> TcS ()
doTopFundepImprovement Ct
work_item
                        ; InertSet -> SimplifierStage
tryLastResortProhibitedSuperclass InertSet
inerts Ct
work_item } }
   where
     dict_loc :: CtLoc
dict_loc = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev


doTopReactDict InertSet
_ Ct
w = String -> SDoc -> TcS (StopOrContinue Ct)
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"doTopReactDict" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
w)

-- | As a last resort, we TEMPORARILY allow a prohibited superclass solve,
-- emitting a loud warning when doing so: we might be creating non-terminating
-- evidence (as we are in T22912 for example).
--
-- See Note [Migrating away from loopy superclass solving] in GHC.Tc.TyCl.Instance.
tryLastResortProhibitedSuperclass :: InertSet -> Ct -> TcS (StopOrContinue Ct)
tryLastResortProhibitedSuperclass :: InertSet -> SimplifierStage
tryLastResortProhibitedSuperclass InertSet
inerts
    work_item :: Ct
work_item@(CDictCan { cc_ev :: Ct -> CtEvidence
cc_ev = CtEvidence
ev_w, cc_class :: Ct -> Class
cc_class = Class
cls, cc_tyargs :: Ct -> [TcPredType]
cc_tyargs = [TcPredType]
xis })
  | let loc_w :: CtLoc
loc_w  = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev_w
        orig_w :: CtOrigin
orig_w = CtLoc -> CtOrigin
ctLocOrigin CtLoc
loc_w
  , ScOrigin ClsInstOrQC
_ NakedScFlag
NakedSc <- CtOrigin
orig_w   -- work_item is definitely Wanted
  , Just Ct
ct_i <- InertCans -> CtLoc -> Class -> [TcPredType] -> Maybe Ct
lookupInertDict (InertSet -> InertCans
inert_cans InertSet
inerts) CtLoc
loc_w Class
cls [TcPredType]
xis
  , let ev_i :: CtEvidence
ev_i = Ct -> CtEvidence
ctEvidence Ct
ct_i
  , CtEvidence -> Bool
isGiven CtEvidence
ev_i
  = do { CtEvidence -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev_w (CtEvidence -> EvTerm
ctEvTerm CtEvidence
ev_i)
       ; CtLoc -> TcRnMessage -> TcS ()
ctLocWarnTcS CtLoc
loc_w (TcRnMessage -> TcS ()) -> TcRnMessage -> TcS ()
forall a b. (a -> b) -> a -> b
$
         CtLoc -> TcPredType -> TcRnMessage
TcRnLoopySuperclassSolve CtLoc
loc_w (Ct -> TcPredType
ctPred Ct
work_item)
       ; StopOrContinue Ct -> TcS (StopOrContinue Ct)
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (StopOrContinue Ct -> TcS (StopOrContinue Ct))
-> StopOrContinue Ct -> TcS (StopOrContinue Ct)
forall a b. (a -> b) -> a -> b
$ CtEvidence -> SDoc -> StopOrContinue Ct
forall a. CtEvidence -> SDoc -> StopOrContinue a
Stop CtEvidence
ev_w (String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Loopy superclass") }
tryLastResortProhibitedSuperclass InertSet
_ Ct
work_item
  = SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
work_item

chooseInstance :: Ct -> ClsInstResult -> TcS (StopOrContinue Ct)
chooseInstance :: Ct -> ClsInstResult -> TcS (StopOrContinue Ct)
chooseInstance Ct
work_item
               (OneInst { cir_new_theta :: ClsInstResult -> [TcPredType]
cir_new_theta = [TcPredType]
theta
                        , cir_what :: ClsInstResult -> InstanceWhat
cir_what      = InstanceWhat
what
                        , cir_mk_ev :: ClsInstResult -> [EvExpr] -> EvTerm
cir_mk_ev     = [EvExpr] -> EvTerm
mk_ev })
  = do { String -> SDoc -> TcS ()
traceTcS String
"doTopReact/found instance for" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$ CtEvidence -> SDoc
forall a. Outputable a => a -> SDoc
ppr CtEvidence
ev
       ; CtLoc
deeper_loc <- CtLoc -> InstanceWhat -> TcPredType -> TcS CtLoc
checkInstanceOK CtLoc
loc InstanceWhat
what TcPredType
pred
       ; CtLoc -> TcPredType -> TcS ()
checkReductionDepth CtLoc
deeper_loc TcPredType
pred
       ; EvBindsVar
evb <- TcS EvBindsVar
getTcEvBindsVar
       ; if EvBindsVar -> Bool
isCoEvBindsVar EvBindsVar
evb
         then SimplifierStage
forall a. a -> TcS (StopOrContinue a)
continueWith Ct
work_item
                  -- See Note [Instances in no-evidence implications]
         else
           do { [MaybeNew]
evc_vars <- (TcPredType -> TcS MaybeNew) -> [TcPredType] -> TcS [MaybeNew]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
mapM (CtLoc -> RewriterSet -> TcPredType -> TcS MaybeNew
newWanted CtLoc
deeper_loc (Ct -> RewriterSet
ctRewriters Ct
work_item)) [TcPredType]
theta
              ; CtEvidence -> EvTerm -> TcS ()
setEvBindIfWanted CtEvidence
ev ([EvExpr] -> EvTerm
mk_ev ((MaybeNew -> EvExpr) -> [MaybeNew] -> [EvExpr]
forall a b. (a -> b) -> [a] -> [b]
map MaybeNew -> EvExpr
getEvExpr [MaybeNew]
evc_vars))
              ; [CtEvidence] -> TcS ()
emitWorkNC ([MaybeNew] -> [CtEvidence]
freshGoals [MaybeNew]
evc_vars)
              ; CtEvidence -> String -> TcS (StopOrContinue Ct)
forall a. CtEvidence -> String -> TcS (StopOrContinue a)
stopWith CtEvidence
ev String
"Dict/Top (solved wanted)" }}
  where
     ev :: CtEvidence
ev         = Ct -> CtEvidence
ctEvidence Ct
work_item
     pred :: TcPredType
pred       = CtEvidence -> TcPredType
ctEvPred CtEvidence
ev
     loc :: CtLoc
loc        = CtEvidence -> CtLoc
ctEvLoc CtEvidence
ev

chooseInstance Ct
work_item ClsInstResult
lookup_res
  = String -> SDoc -> TcS (StopOrContinue Ct)
forall a. HasCallStack => String -> SDoc -> a
pprPanic String
"chooseInstance" (Ct -> SDoc
forall a. Outputable a => a -> SDoc
ppr Ct
work_item SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ ClsInstResult -> SDoc
forall a. Outputable a => a -> SDoc
ppr ClsInstResult
lookup_res)

checkInstanceOK :: CtLoc -> InstanceWhat -> TcPredType -> TcS CtLoc
-- Check that it's OK to use this instance:
--    (a) the use is well staged in the Template Haskell sense
-- Returns the CtLoc to used for sub-goals
-- Probably also want to call checkReductionDepth
checkInstanceOK :: CtLoc -> InstanceWhat -> TcPredType -> TcS CtLoc
checkInstanceOK CtLoc
loc InstanceWhat
what TcPredType
pred
  = do { CtLoc -> InstanceWhat -> TcPredType -> TcS ()
checkWellStagedDFun CtLoc
loc InstanceWhat
what TcPredType
pred
       ; CtLoc -> TcS CtLoc
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return CtLoc
deeper_loc }
  where
     deeper_loc :: CtLoc
deeper_loc = CtLoc -> CtLoc
zap_origin (CtLoc -> CtLoc
bumpCtLocDepth CtLoc
loc)
     origin :: CtOrigin
origin     = CtLoc -> CtOrigin
ctLocOrigin CtLoc
loc

     zap_origin :: CtLoc -> CtLoc
zap_origin CtLoc
loc  -- After applying an instance we can set ScOrigin to
                     -- NotNakedSc, so that prohibitedSuperClassSolve never fires
                     -- See Note [Solving superclass constraints] in
                     -- GHC.Tc.TyCl.Instance, (sc1).
       | ScOrigin ClsInstOrQC
what NakedScFlag
_ <- CtOrigin
origin
       = CtLoc -> CtOrigin -> CtLoc
setCtLocOrigin CtLoc
loc (ClsInstOrQC -> NakedScFlag -> CtOrigin
ScOrigin ClsInstOrQC
what NakedScFlag
NotNakedSc)
       | Bool
otherwise
       = CtLoc
loc

{- Note [Instances in no-evidence implications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In #15290 we had
  [G] forall p q. Coercible p q => Coercible (m p) (m q))
  [W] forall <no-ev> a. m (Int, IntStateT m a)
                          ~R#
                        m (Int, StateT Int m a)

The Given is an ordinary quantified constraint; the Wanted is an implication
equality that arises from
  [W] (forall a. t1) ~R# (forall a. t2)

But because the (t1 ~R# t2) is solved "inside a type" (under that forall a)
we can't generate any term evidence.  So we can't actually use that
lovely quantified constraint.  Alas!

This test arranges to ignore the instance-based solution under these
(rare) circumstances.   It's sad, but I  really don't see what else we can do.
-}


matchClassInst :: DynFlags -> InertSet
               -> Class -> [Type]
               -> CtLoc -> TcS ClsInstResult
matchClassInst :: DynFlags
-> InertSet -> Class -> [TcPredType] -> CtLoc -> TcS ClsInstResult
matchClassInst DynFlags
dflags InertSet
inerts Class
clas [TcPredType]
tys CtLoc
loc
-- First check whether there is an in-scope Given that could
-- match this constraint.  In that case, do not use any instance
-- whether top level, or local quantified constraints.
-- See Note [Instance and Given overlap]
  | Bool -> Bool
not (Extension -> DynFlags -> Bool
xopt Extension
LangExt.IncoherentInstances DynFlags
dflags)
  , Bool -> Bool
not (Class -> Bool
naturallyCoherentClass Class
clas)
  , Bool -> Bool
not (InertSet -> CtLoc -> Class -> [TcPredType] -> Bool
noMatchableGivenDicts InertSet
inerts CtLoc
loc Class
clas [TcPredType]
tys)
  = do { String -> SDoc -> TcS ()
traceTcS String
"Delaying instance application" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
           [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"Work item=" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Class -> [TcPredType] -> SDoc
pprClassPred Class
clas [TcPredType]
tys ]
       ; ClsInstResult -> TcS ClsInstResult
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
NotSure }

  | Bool
otherwise
  = do { String -> SDoc -> TcS ()
traceTcS String
"matchClassInst" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"pred =" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcPredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcPredType
pred SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Char -> SDoc
forall doc. IsLine doc => Char -> doc
char Char
'{'
       ; ClsInstResult
local_res <- TcPredType -> CtLoc -> TcS ClsInstResult
matchLocalInst TcPredType
pred CtLoc
loc
       ; case ClsInstResult
local_res of
           OneInst {} ->  -- See Note [Local instances and incoherence]
                do { String -> SDoc -> TcS ()
traceTcS String
"} matchClassInst local match" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$ ClsInstResult -> SDoc
forall a. Outputable a => a -> SDoc
ppr ClsInstResult
local_res
                   ; ClsInstResult -> TcS ClsInstResult
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
local_res }

           ClsInstResult
NotSure -> -- In the NotSure case for local instances
                      -- we don't want to try global instances
                do { String -> SDoc -> TcS ()
traceTcS String
"} matchClassInst local not sure" SDoc
forall doc. IsOutput doc => doc
empty
                   ; ClsInstResult -> TcS ClsInstResult
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
local_res }

           ClsInstResult
NoInstance  -- No local instances, so try global ones
              -> do { ClsInstResult
global_res <- DynFlags -> Bool -> Class -> [TcPredType] -> TcS ClsInstResult
matchGlobalInst DynFlags
dflags Bool
False Class
clas [TcPredType]
tys
                    ; String -> SDoc -> TcS ()
traceTcS String
"} matchClassInst global result" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$ ClsInstResult -> SDoc
forall a. Outputable a => a -> SDoc
ppr ClsInstResult
global_res
                    ; ClsInstResult -> TcS ClsInstResult
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
global_res } }
  where
    pred :: TcPredType
pred = Class -> [TcPredType] -> TcPredType
mkClassPred Class
clas [TcPredType]
tys

-- | If a class is "naturally coherent", then we needn't worry at all, in any
-- way, about overlapping/incoherent instances. Just solve the thing!
-- See Note [Naturally coherent classes]
-- See also Note [The equality types story] in GHC.Builtin.Types.Prim.
naturallyCoherentClass :: Class -> Bool
naturallyCoherentClass :: Class -> Bool
naturallyCoherentClass Class
cls
  = Class -> Bool
isCTupleClass Class
cls
    Bool -> Bool -> Bool
|| Class
cls Class -> Unique -> Bool
forall a. Uniquable a => a -> Unique -> Bool
`hasKey` Unique
heqTyConKey
    Bool -> Bool -> Bool
|| Class
cls Class -> Unique -> Bool
forall a. Uniquable a => a -> Unique -> Bool
`hasKey` Unique
eqTyConKey
    Bool -> Bool -> Bool
|| Class
cls Class -> Unique -> Bool
forall a. Uniquable a => a -> Unique -> Bool
`hasKey` Unique
coercibleTyConKey


{- Note [Instance and Given overlap]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Example, from the OutsideIn(X) paper:
       instance P x => Q [x]
       instance (x ~ y) => R y [x]

       wob :: forall a b. (Q [b], R b a) => a -> Int

       g :: forall a. Q [a] => [a] -> Int
       g x = wob x

From 'g' we get the implication constraint:
            forall a. Q [a] => (Q [beta], R beta [a])
If we react (Q [beta]) with its top-level axiom, we end up with a
(P beta), which we have no way of discharging. On the other hand,
if we react R beta [a] with the top-level we get  (beta ~ a), which
is solvable and can help us rewrite (Q [beta]) to (Q [a]) which is
now solvable by the given Q [a].

The partial solution is that:
  In matchClassInst (and thus in topReact), we return a matching
  instance only when there is no Given in the inerts which is
  unifiable to this particular dictionary.

  We treat any meta-tyvar as "unifiable" for this purpose,
  *including* untouchable ones.  But not skolems like 'a' in
  the implication constraint above.

The end effect is that, much as we do for overlapping instances, we
delay choosing a class instance if there is a possibility of another
instance OR a given to match our constraint later on. This fixes
tickets #4981 and #5002.

Other notes:

* The check is done *first*, so that it also covers classes
  with built-in instance solving, such as
     - constraint tuples
     - natural numbers
     - Typeable

* See also Note [What might equal later?] in GHC.Tc.Solver.InertSet.

* The given-overlap problem is arguably not easy to appear in practice
  due to our aggressive prioritization of equality solving over other
  constraints, but it is possible. I've added a test case in
  typecheck/should-compile/GivenOverlapping.hs

* Another "live" example is #10195; another is #10177.

* We ignore the overlap problem if -XIncoherentInstances is in force:
  see #6002 for a worked-out example where this makes a
  difference.

* Moreover notice that our goals here are different than the goals of
  the top-level overlapping checks. There we are interested in
  validating the following principle:

      If we inline a function f at a site where the same global
      instance environment is available as the instance environment at
      the definition site of f then we should get the same behaviour.

  But for the Given Overlap check our goal is just related to completeness of
  constraint solving.

* The solution is only a partial one.  Consider the above example with
       g :: forall a. Q [a] => [a] -> Int
       g x = let v = wob x
             in v
  and suppose we have -XNoMonoLocalBinds, so that we attempt to find the most
  general type for 'v'.  When generalising v's type we'll simplify its
  Q [alpha] constraint, but we don't have Q [a] in the 'givens', so we
  will use the instance declaration after all. #11948 was a case
  in point.

All of this is disgustingly delicate, so to discourage people from writing
simplifiable class givens, we warn about signatures that contain them;
see GHC.Tc.Validity Note [Simplifiable given constraints].

Note [Naturally coherent classes]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A few built-in classes are "naturally coherent".  This term means that
the "instance" for the class is bidirectional with its superclass(es).
For example, consider (~~), which behaves as if it was defined like
this:
  class a ~# b => a ~~ b
  instance a ~# b => a ~~ b
(See Note [The equality types story] in GHC.Builtin.Types.Prim.)

Faced with [W] t1 ~~ t2, it's always OK to reduce it to [W] t1 ~# t2,
without worrying about Note [Instance and Given overlap].  Why?  Because
if we had [G] s1 ~~ s2, then we'd get the superclass [G] s1 ~# s2, and
so the reduction of the [W] constraint does not risk losing any solutions.

On the other hand, it can be fatal to /fail/ to reduce such
equalities, on the grounds of Note [Instance and Given overlap],
because many good things flow from [W] t1 ~# t2.

The same reasoning applies to

* (~~)        heqTyCon
* (~)         eqTyCon
* Coercible   coercibleTyCon

And less obviously to:

* Tuple classes.  For reasons described in GHC.Tc.Solver.Types
  Note [Tuples hiding implicit parameters], we may have a constraint
     [W] (?x::Int, C a)
  with an exactly-matching Given constraint.  We must decompose this
  tuple and solve the components separately, otherwise we won't solve
  it at all!  It is perfectly safe to decompose it, because again the
  superclasses invert the instance;  e.g.
      class (c1, c2) => (% c1, c2 %)
      instance (c1, c2) => (% c1, c2 %)
  Example in #14218

Examples: T5853, T10432, T5315, T9222, T2627b, T3028b

PS: the term "naturally coherent" doesn't really seem helpful.
Perhaps "invertible" or something?  I left it for now though.

Note [Local instances and incoherence]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
   f :: forall b c. (Eq b, forall a. Eq a => Eq (c a))
                 => c b -> Bool
   f x = x==x

We get [W] Eq (c b), and we must use the local instance to solve it.

BUT that wanted also unifies with the top-level Eq [a] instance,
and Eq (Maybe a) etc.  We want the local instance to "win", otherwise
we can't solve the wanted at all.  So we mark it as Incohherent.
According to Note [Rules for instance lookup] in GHC.Core.InstEnv, that'll
make it win even if there are other instances that unify.

Moreover this is not a hack!  The evidence for this local instance
will be constructed by GHC at a call site... from the very instances
that unify with it here.  It is not like an incoherent user-written
instance which might have utterly different behaviour.

Consider  f :: Eq a => blah.  If we have [W] Eq a, we certainly
get it from the Eq a context, without worrying that there are
lots of top-level instances that unify with [W] Eq a!  We'll use
those instances to build evidence to pass to f. That's just the
nullary case of what's happening here.
-}

matchLocalInst :: TcPredType -> CtLoc -> TcS ClsInstResult
-- Look up the predicate in Given quantified constraints,
-- which are effectively just local instance declarations.
matchLocalInst :: TcPredType -> CtLoc -> TcS ClsInstResult
matchLocalInst TcPredType
pred CtLoc
loc
  = do { inerts :: InertSet
inerts@(IS { inert_cans :: InertSet -> InertCans
inert_cans = InertCans
ics }) <- TcS InertSet
getTcSInerts
       ; case InertSet
-> [QCInst]
-> ([(CtEvidence, [DFunInstType])], [(CtEvidence, [DFunInstType])])
match_local_inst InertSet
inerts (InertCans -> [QCInst]
inert_insts InertCans
ics) of
          { ([], []) -> do { String -> SDoc -> TcS ()
traceTcS String
"No local instance for" (TcPredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcPredType
pred)
                           ; ClsInstResult -> TcS ClsInstResult
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
NoInstance }
          ; ([(CtEvidence, [DFunInstType])]
matches, [(CtEvidence, [DFunInstType])]
unifs) ->
    do { [InstDFun]
matches <- ((CtEvidence, [DFunInstType]) -> TcS InstDFun)
-> [(CtEvidence, [DFunInstType])] -> TcS [InstDFun]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
mapM (CtEvidence, [DFunInstType]) -> TcS InstDFun
mk_instDFun [(CtEvidence, [DFunInstType])]
matches
       ; [InstDFun]
unifs   <- ((CtEvidence, [DFunInstType]) -> TcS InstDFun)
-> [(CtEvidence, [DFunInstType])] -> TcS [InstDFun]
forall (t :: * -> *) (m :: * -> *) a b.
(Traversable t, Monad m) =>
(a -> m b) -> t a -> m (t b)
forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
mapM (CtEvidence, [DFunInstType]) -> TcS InstDFun
mk_instDFun [(CtEvidence, [DFunInstType])]
unifs
         -- See Note [Use only the best matching quantified constraint]
       ; case [InstDFun] -> Maybe InstDFun
dominatingMatch [InstDFun]
matches of
          { Just (TyVar
dfun_id, [TcPredType]
tys, [TcPredType]
theta)
            | (InstDFun -> Bool) -> [InstDFun] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
all (([TcPredType]
theta [TcPredType] -> [TcPredType] -> Bool
`impliedBySCs`) ([TcPredType] -> Bool)
-> (InstDFun -> [TcPredType]) -> InstDFun -> Bool
forall b c a. (b -> c) -> (a -> b) -> a -> c
. InstDFun -> [TcPredType]
forall a b c. (a, b, c) -> c
thdOf3) [InstDFun]
unifs
            ->
            do { let result :: ClsInstResult
result = OneInst { cir_new_theta :: [TcPredType]
cir_new_theta = [TcPredType]
theta
                                      , cir_mk_ev :: [EvExpr] -> EvTerm
cir_mk_ev     = TyVar -> [TcPredType] -> [EvExpr] -> EvTerm
evDFunApp TyVar
dfun_id [TcPredType]
tys
                                      , cir_what :: InstanceWhat
cir_what      = InstanceWhat
LocalInstance }
               ; String -> SDoc -> TcS ()
traceTcS String
"Best local instance found:" (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$
                  [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"pred:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcPredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcPredType
pred
                       , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"result:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> ClsInstResult -> SDoc
forall a. Outputable a => a -> SDoc
ppr ClsInstResult
result
                       , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"matches:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [InstDFun] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [InstDFun]
matches
                       , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"unifs:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [InstDFun] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [InstDFun]
unifs ]
               ; ClsInstResult -> TcS ClsInstResult
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
result }

          ; Maybe InstDFun
mb_best ->
            do { String -> SDoc -> TcS ()
traceTcS String
"Multiple local instances; not committing to any"
                  (SDoc -> TcS ()) -> SDoc -> TcS ()
forall a b. (a -> b) -> a -> b
$ [SDoc] -> SDoc
forall doc. IsDoc doc => [doc] -> doc
vcat [ String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"pred:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> TcPredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcPredType
pred
                         , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"matches:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [InstDFun] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [InstDFun]
matches
                         , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"unifs:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> [InstDFun] -> SDoc
forall a. Outputable a => a -> SDoc
ppr [InstDFun]
unifs
                         , String -> SDoc
forall doc. IsLine doc => String -> doc
text String
"best_match:" SDoc -> SDoc -> SDoc
forall doc. IsLine doc => doc -> doc -> doc
<+> Maybe InstDFun -> SDoc
forall a. Outputable a => a -> SDoc
ppr Maybe InstDFun
mb_best ]
               ; ClsInstResult -> TcS ClsInstResult
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return ClsInstResult
NotSure }}}}}
  where
    pred_tv_set :: VarSet
pred_tv_set = TcPredType -> VarSet
tyCoVarsOfType TcPredType
pred

    mk_instDFun :: (CtEvidence, [DFunInstType]) -> TcS InstDFun
    mk_instDFun :: (CtEvidence, [DFunInstType]) -> TcS InstDFun
mk_instDFun (CtEvidence
ev, [DFunInstType]
tys) =
      let dfun_id :: TyVar
dfun_id = CtEvidence -> TyVar
ctEvEvId CtEvidence
ev
      in do { ([TcPredType]
tys, [TcPredType]
theta) <- TyVar -> [DFunInstType] -> TcS ([TcPredType], [TcPredType])
instDFunType (CtEvidence -> TyVar
ctEvEvId CtEvidence
ev) [DFunInstType]
tys
            ; InstDFun -> TcS InstDFun
forall a. a -> TcS a
forall (m :: * -> *) a. Monad m => a -> m a
return (TyVar
dfun_id, [TcPredType]
tys, [TcPredType]
theta) }

    -- Compute matching and unifying local instances
    match_local_inst :: InertSet
                     -> [QCInst]
                     -> ( [(CtEvidence, [DFunInstType])]
                        , [(CtEvidence, [DFunInstType])] )
    match_local_inst :: InertSet
-> [QCInst]
-> ([(CtEvidence, [DFunInstType])], [(CtEvidence, [DFunInstType])])
match_local_inst InertSet
_inerts []
      = ([], [])
    match_local_inst InertSet
inerts (qci :: QCInst
qci@(QCI { qci_tvs :: QCInst -> [TyVar]
qci_tvs  = [TyVar]
qtvs
                                      , qci_pred :: QCInst -> TcPredType
qci_pred = TcPredType
qpred
                                      , qci_ev :: QCInst -> CtEvidence
qci_ev   = CtEvidence
qev })
                            :[QCInst]
qcis)
      | let in_scope :: InScopeSet
in_scope = VarSet -> InScopeSet
mkInScopeSet (VarSet
qtv_set VarSet -> VarSet -> VarSet
`unionVarSet` VarSet
pred_tv_set)
      , Just VarEnv TcPredType
tv_subst <- VarSet
-> RnEnv2
-> VarEnv TcPredType
-> TcPredType
-> TcPredType
-> Maybe (VarEnv TcPredType)
ruleMatchTyKiX VarSet
qtv_set (InScopeSet -> RnEnv2
mkRnEnv2 InScopeSet
in_scope)
                                        VarEnv TcPredType
emptyTvSubstEnv TcPredType
qpred TcPredType
pred
      , let match :: (CtEvidence, [DFunInstType])
match = (CtEvidence
qev, (TyVar -> DFunInstType) -> [TyVar] -> [DFunInstType]
forall a b. (a -> b) -> [a] -> [b]
map (VarEnv TcPredType -> TyVar -> DFunInstType
forall a. VarEnv a -> TyVar -> Maybe a
lookupVarEnv VarEnv TcPredType
tv_subst) [TyVar]
qtvs)
      = ((CtEvidence, [DFunInstType])
match(CtEvidence, [DFunInstType])
-> [(CtEvidence, [DFunInstType])] -> [(CtEvidence, [DFunInstType])]
forall a. a -> [a] -> [a]
:[(CtEvidence, [DFunInstType])]
matches, [(CtEvidence, [DFunInstType])]
unifs)

      | Bool
otherwise
      = Bool
-> SDoc
-> ([(CtEvidence, [DFunInstType])], [(CtEvidence, [DFunInstType])])
-> ([(CtEvidence, [DFunInstType])], [(CtEvidence, [DFunInstType])])
forall a. HasCallStack => Bool -> SDoc -> a -> a
assertPpr (VarSet -> VarSet -> Bool
disjointVarSet VarSet
qtv_set (TcPredType -> VarSet
tyCoVarsOfType TcPredType
pred))
                  (QCInst -> SDoc
forall a. Outputable a => a -> SDoc
ppr QCInst
qci SDoc -> SDoc -> SDoc
forall doc. IsDoc doc => doc -> doc -> doc
$$ TcPredType -> SDoc
forall a. Outputable a => a -> SDoc
ppr TcPredType
pred)
            -- ASSERT: unification relies on the
            -- quantified variables being fresh
        ([(CtEvidence, [DFunInstType])]
matches, Maybe (CtEvidence, [DFunInstType])
this_unif Maybe (CtEvidence, [DFunInstType])
-> [(CtEvidence, [DFunInstType])] -> [(CtEvidence, [DFunInstType])]
forall {a}. Maybe a -> [a] -> [a]
`combine` [(CtEvidence, [DFunInstType])]
unifs)
      where
        qloc :: CtLoc
qloc = CtEvidence -> CtLoc
ctEvLoc CtEvidence
qev
        qtv_set :: VarSet
qtv_set = [TyVar] -> VarSet
mkVarSet [TyVar]
qtvs
        ([(CtEvidence, [DFunInstType])]
matches, [(CtEvidence, [DFunInstType])]
unifs) = InertSet
-> [QCInst]
-> ([(CtEvidence, [DFunInstType])], [(CtEvidence, [DFunInstType])])
match_local_inst InertSet
inerts [QCInst]
qcis
        this_unif :: Maybe (CtEvidence, [DFunInstType])
this_unif
          | Just Subst
subst <- InertSet
-> TcPredType -> CtLoc -> TcPredType -> CtLoc -> Maybe Subst
mightEqualLater InertSet
inerts TcPredType
qpred CtLoc
qloc TcPredType
pred CtLoc
loc
          = (CtEvidence, [DFunInstType]) -> Maybe (CtEvidence, [DFunInstType])
forall a. a -> Maybe a
Just (CtEvidence
qev, (TyVar -> DFunInstType) -> [TyVar] -> [DFunInstType]
forall a b. (a -> b) -> [a] -> [b]
map  (Subst -> TyVar -> DFunInstType
lookupTyVar Subst
subst) [TyVar]
qtvs)
          | Bool
otherwise
          = Maybe (CtEvidence, [DFunInstType])
forall a. Maybe a
Nothing

        combine :: Maybe a -> [a] -> [a]
combine Maybe a
Nothing  [a]
us = [a]
us
        combine (Just a
u) [a]
us = a
u a -> [a] -> [a]
forall a. a -> [a] -> [a]
: [a]
us

-- | Instance dictionary function and type.
type InstDFun = (DFunId, [TcType], TcThetaType)

-- | Try to find a local quantified instance that dominates all others,
-- i.e. which has a weaker instance context than all the others.
--
-- See Note [Use only the best matching quantified constraint].
dominatingMatch :: [InstDFun] -> Maybe InstDFun
dominatingMatch :: [InstDFun] -> Maybe InstDFun
dominatingMatch [InstDFun]
matches =
  [InstDFun] -> Maybe InstDFun
forall a. [a] -> Maybe a
listToMaybe ([InstDFun] -> Maybe InstDFun) -> [InstDFun] -> Maybe InstDFun
forall a b. (a -> b) -> a -> b
$ ((InstDFun, [InstDFun]) -> Maybe InstDFun)
-> [(InstDFun, [InstDFun])] -> [InstDFun]
forall a b. (a -> Maybe b) -> [a] -> [b]
mapMaybe ((InstDFun -> [InstDFun] -> Maybe InstDFun)
-> (InstDFun, [InstDFun]) -> Maybe InstDFun
forall a b c. (a -> b -> c) -> (a, b) -> c
uncurry InstDFun -> [InstDFun] -> Maybe InstDFun
go) ([InstDFun] -> [(InstDFun, [InstDFun])]
forall a. [a] -> [(a, [a])]
holes [InstDFun]
matches)
  -- listToMaybe: arbitrarily pick any one context that is weaker than
  -- all others, e.g. so that we can handle [Eq a, Num a] vs [Num a, Eq a]
  -- (see test case T22223).

  where
    go :: InstDFun -> [InstDFun] -> Maybe InstDFun
    go :: InstDFun -> [InstDFun] -> Maybe InstDFun
go InstDFun
this [] = InstDFun -> Maybe InstDFun
forall a. a -> Maybe a
Just InstDFun
this
    go this :: InstDFun
this@(TyVar
_,[TcPredType]
_,[TcPredType]
this_theta) ((TyVar
_,[TcPredType]
_,[TcPredType]
other_theta):[InstDFun]
others)
      | [TcPredType]
this_theta [TcPredType] -> [TcPredType] -> Bool
`impliedBySCs` [TcPredType]
other_theta
      = InstDFun -> [InstDFun] -> Maybe InstDFun
go InstDFun
this [InstDFun]
others
      | Bool
otherwise
      = Maybe InstDFun
forall a. Maybe a
Nothing

-- | Whether a collection of constraints is implied by another collection,
-- according to a simple superclass check.
--
-- See Note [When does a quantified instance dominate another?].
impliedBySCs :: TcThetaType -> TcThetaType -> Bool
impliedBySCs :: [TcPredType] -> [TcPredType] -> Bool
impliedBySCs [TcPredType]
c1 [TcPredType]
c2 = (TcPredType -> Bool) -> [TcPredType] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
all TcPredType -> Bool
in_c2 [TcPredType]
c1
  where
    in_c2 :: TcPredType -> Bool
    in_c2 :: TcPredType -> Bool
in_c2 TcPredType
pred = (TcPredType -> Bool) -> [TcPredType] -> Bool
forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
any (TcPredType
pred (() :: Constraint) => TcPredType -> TcPredType -> Bool
TcPredType -> TcPredType -> Bool
`tcEqType`) [TcPredType]
c2_expanded

    c2_expanded :: [TcPredType]  -- Includes all superclasses
    c2_expanded :: [TcPredType]
c2_expanded = [ TcPredType
q | TcPredType
p <- [TcPredType]
c2, TcPredType
q <- TcPredType
p TcPredType -> [TcPredType] -> [TcPredType]
forall a. a -> [a] -> [a]
: TcPredType -> [TcPredType]
transSuperClasses TcPredType
p ]


{- Note [When does a quantified instance dominate another?]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When matching local quantified instances, it's useful to be able to pick
the one with the weakest precondition, e.g. if one has both

  [G] d1: forall a b. ( Eq a, Num b, C a b  ) => D a b
  [G] d2: forall a  .                C a Int  => D a Int
  [W] {w}: D a Int

Then it makes sense to use d2 to solve w, as doing so we end up with a strictly
weaker proof obligation of `C a Int`, compared to `(Eq a, Num Int, C a Int)`
were we to use d1.

In theory, to compute whether one context implies another, we would need to
recursively invoke the constraint solver. This is expensive, so we instead do
a simple check using superclasses, implemented in impliedBySCs.

Examples:

 - [Eq a] is implied by [Ord a]
 - [Ord a] is not implied by [Eq a],
 - any context is implied by itself,
 - the empty context is implied by any context.

Note [Use only the best matching quantified constraint]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider (#20582) the ambiguity check for
  (forall a. Ord (m a), forall a. Semigroup a => Eq (m a)) => m Int

Because of eager expansion of given superclasses, we get
  [G] d1: forall a. Ord (m a)
  [G] d2: forall a. Eq (m a)
  [G] d3: forall a. Semigroup a => Eq (m a)

  [W] {w1}: forall a. Ord (m a)
  [W] {w2}: forall a. Semigroup a => Eq (m a)

The first wanted is solved straightforwardly. But the second wanted
matches *two* local instances: d2 and d3. Our general rule around multiple local
instances is that we refuse to commit to any of them. However, that
means that our type fails the ambiguity check. That's bad: the type
is perfectly fine. (This actually came up in the wild, in the streamly
library.)

The solution is to prefer local instances which are easier to prove, meaning
that they have a weaker precondition. In this case, the empty context
of d2 is a weaker constraint than the "Semigroup a" context of d3, so we prefer
using it when proving w2. This allows us to pass the ambiguity check here.

Our criterion for solving a Wanted by matching local quantified instances is
thus as follows:

  - There is a matching local quantified instance that dominates all others
    matches, in the sense of [When does a quantified instance dominate another?].
    Any such match do, we pick it arbitrarily (the T22223 example below says why).
  - This local quantified instance also dominates all the unifiers, as we
    wouldn't want to commit to a single match when we might have multiple,
    genuinely different matches after further unification takes place.

Some other examples:


  #15244:

    f :: (C g, D g) => ....
    class S g => C g where ...
    class S g => D g where ...
    class (forall a. Eq a => Eq (g a)) => S g where ...

  Here, in f's RHS, there are two identical quantified constraints
  available, one via the superclasses of C and one via the superclasses
  of D. Given that each implies the other, we pick one arbitrarily.


  #22216:

    class Eq a
    class Eq a => Ord a
    class (forall b. Eq b => Eq (f b)) => Eq1 f
    class (Eq1 f, forall b. Ord b => Ord (f b)) => Ord1 f

  Suppose we have

    [G] d1: Ord1 f
    [G] d2: Eq a
    [W] {w}: Eq (f a)

  Superclass expansion of d1 gives us:

    [G] d3 : Eq1 f
    [G] d4 : forall b. Ord b => Ord (f b)

  expanding d4 and d5 gives us, respectively:

    [G] d5 : forall b. Eq  b => Eq (f b)
    [G] d6 : forall b. Ord b => Eq (f b)

  Now we have two matching local instances that we could use when solving the
  Wanted. However, it's obviously silly to use d6, given that d5 provides us with
  as much information, with a strictly weaker precondition. So we pick d5 to solve
  w. If we chose d6, we would get [W] Ord a, which in this case we can't solve.


  #22223:

    [G] forall a b. (Eq a, Ord b) => C a b
    [G] forall a b. (Ord b, Eq a) => C a b
    [W] C x y

  Here we should be free to pick either quantified constraint, as they are
  equivalent up to re-ordering of the constraints in the context.
  See also Note [Do not add duplicate quantified instances]
  in GHC.Tc.Solver.Monad.

Test cases:
  typecheck/should_compile/T20582
  quantified-constraints/T15244
  quantified-constraints/T22216{a,b,c,d,e}
  quantified-constraints/T22223

Historical note: a previous solution was to instead pick the local instance
with the least superclass depth (see Note [Replacement vs keeping]),
but that doesn't work for the example from #22216.
-}