TcSimplify.hs 84.2 KB
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{-# LANGUAGE CPP #-}

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module TcSimplify(
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       simplifyInfer,
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       growThetaTyVars,
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       simplifyAmbiguityCheck,
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       simplifyDefault,
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       simplifyTop, simplifyInteractive, solveEqualities,
       simplifyWantedsTcM,
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       tcCheckSatisfiability,
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       -- For Rules we need these
       solveWanteds, runTcSDeriveds
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  ) where
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#include "HsVersions.h"
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import Bag
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import Class         ( Class, classKey, classTyCon )
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import DynFlags      ( WarningFlag ( Opt_WarnMonomorphism )
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                     , DynFlags( solverIterations ) )
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import Inst
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import ListSetOps
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import Maybes
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import Name
import Outputable
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import Pair
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import PrelInfo
import PrelNames
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import TcErrors
import TcEvidence
import TcInteract
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import TcCanonical   ( makeSuperClasses, mkGivensWithSuperClasses )
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import TcMType   as TcM
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import TcRnMonad as TcM
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import TcSMonad  as TcS
import TcType
import TrieMap       () -- DV: for now
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import Type
import TysWiredIn    ( liftedDataConTy )
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import Unify         ( tcMatchTy )
import Util
import Var
import VarSet
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import BasicTypes    ( IntWithInf, intGtLimit )
import ErrUtils      ( emptyMessages )
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import qualified GHC.LanguageExtensions as LangExt
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import Control.Monad ( when, unless )
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import Data.List     ( partition )
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import Data.Foldable    ( fold )

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{-
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*********************************************************************************
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*                                                                               *
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*                           External interface                                  *
*                                                                               *
*********************************************************************************
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-}
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simplifyTop :: WantedConstraints -> TcM (Bag EvBind)
-- Simplify top-level constraints
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-- Usually these will be implications,
-- but when there is nothing to quantify we don't wrap
-- in a degenerate implication, so we do that here instead
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simplifyTop wanteds
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  = do { traceTc "simplifyTop {" $ text "wanted = " <+> ppr wanteds
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       ; ((final_wc, unsafe_ol), binds1) <- runTcS $
            do { final_wc <- simpl_top wanteds
               ; unsafe_ol <- getSafeOverlapFailures
               ; return (final_wc, unsafe_ol) }
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       ; traceTc "End simplifyTop }" empty

       ; traceTc "reportUnsolved {" empty
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       ; binds2 <- reportUnsolved final_wc
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       ; traceTc "reportUnsolved }" empty
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       ; traceTc "reportUnsolved (unsafe overlapping) {" empty
       ; unless (isEmptyCts unsafe_ol) $ do {
           -- grab current error messages and clear, warnAllUnsolved will
           -- update error messages which we'll grab and then restore saved
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           -- messages.
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           ; errs_var  <- getErrsVar
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           ; saved_msg <- TcM.readTcRef errs_var
           ; TcM.writeTcRef errs_var emptyMessages
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           ; warnAllUnsolved $ WC { wc_simple = unsafe_ol
                                  , wc_insol = emptyCts
                                  , wc_impl = emptyBag }

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           ; whyUnsafe <- fst <$> TcM.readTcRef errs_var
           ; TcM.writeTcRef errs_var saved_msg
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           ; recordUnsafeInfer whyUnsafe
           }
       ; traceTc "reportUnsolved (unsafe overlapping) }" empty

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       ; return (evBindMapBinds binds1 `unionBags` binds2) }

-- | Type-check a thing that emits only equality constraints, then
-- solve those constraints. Emits errors -- but does not fail --
-- if there is trouble.
solveEqualities :: TcM a -> TcM a
solveEqualities thing_inside
  = do { (result, wanted) <- captureConstraints thing_inside
       ; traceTc "solveEqualities {" $ text "wanted = " <+> ppr wanted
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       ; final_wc <- runTcSEqualities $ simpl_top wanted
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       ; traceTc "End solveEqualities }" empty

       ; traceTc "reportAllUnsolved {" empty
       ; reportAllUnsolved final_wc
       ; traceTc "reportAllUnsolved }" empty
       ; return result }
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simpl_top :: WantedConstraints -> TcS WantedConstraints
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    -- See Note [Top-level Defaulting Plan]
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simpl_top wanteds
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  = do { wc_first_go <- nestTcS (solveWantedsAndDrop wanteds)
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                            -- This is where the main work happens
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       ; try_tyvar_defaulting wc_first_go }
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  where
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    try_tyvar_defaulting :: WantedConstraints -> TcS WantedConstraints
    try_tyvar_defaulting wc
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      | isEmptyWC wc
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      = return wc
      | otherwise
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      = do { free_tvs <- TcS.zonkTyCoVarsAndFV (tyCoVarsOfWC wc)
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           ; let meta_tvs = varSetElems (filterVarSet isMetaTyVar free_tvs)
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                   -- zonkTyCoVarsAndFV: the wc_first_go is not yet zonked
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                   -- filter isMetaTyVar: we might have runtime-skolems in GHCi,
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                   -- and we definitely don't want to try to assign to those!

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           ; defaulted <- mapM defaultTyVarTcS meta_tvs   -- Has unification side effects
           ; if or defaulted
             then do { wc_residual <- nestTcS (solveWanteds wc)
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                            -- See Note [Must simplify after defaulting]
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                     ; try_class_defaulting wc_residual }
             else try_class_defaulting wc }     -- No defaulting took place
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    try_class_defaulting :: WantedConstraints -> TcS WantedConstraints
    try_class_defaulting wc
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      | isEmptyWC wc
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      = return wc
      | otherwise  -- See Note [When to do type-class defaulting]
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      = do { something_happened <- applyDefaultingRules wc
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                                   -- See Note [Top-level Defaulting Plan]
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           ; if something_happened
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             then do { wc_residual <- nestTcS (solveWantedsAndDrop wc)
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                     ; try_class_defaulting wc_residual }
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                  -- See Note [Overview of implicit CallStacks]
             else try_callstack_defaulting wc }

    try_callstack_defaulting :: WantedConstraints -> TcS WantedConstraints
    try_callstack_defaulting wc
      | isEmptyWC wc
      = return wc
      | otherwise
      = defaultCallStacks wc

-- | Default any remaining @CallStack@ constraints to empty @CallStack@s.
defaultCallStacks :: WantedConstraints -> TcS WantedConstraints
-- See Note [Overview of implicit CallStacks]
defaultCallStacks wanteds
  = do simples <- handle_simples (wc_simple wanteds)
       implics <- mapBagM handle_implic (wc_impl wanteds)
       return (wanteds { wc_simple = simples, wc_impl = implics })

  where

  handle_simples simples
    = catBagMaybes <$> mapBagM defaultCallStack simples

  handle_implic implic = do
    wanteds <- defaultCallStacks (ic_wanted implic)
    return (implic { ic_wanted = wanteds })

  defaultCallStack ct@(CDictCan { cc_ev = ev_w })
    | Just _ <- isCallStackCt ct
    = do { solveCallStack ev_w EvCsEmpty
         ; return Nothing }

  defaultCallStack ct
    = return (Just ct)

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{-
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Note [When to do type-class defaulting]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In GHC 7.6 and 7.8.2, we did type-class defaulting only if insolubleWC
was false, on the grounds that defaulting can't help solve insoluble
constraints.  But if we *don't* do defaulting we may report a whole
lot of errors that would be solved by defaulting; these errors are
quite spurious because fixing the single insoluble error means that
defaulting happens again, which makes all the other errors go away.
This is jolly confusing: Trac #9033.

So it seems better to always do type-class defaulting.

However, always doing defaulting does mean that we'll do it in
situations like this (Trac #5934):
   run :: (forall s. GenST s) -> Int
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   run = fromInteger 0
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We don't unify the return type of fromInteger with the given function
type, because the latter involves foralls.  So we're left with
    (Num alpha, alpha ~ (forall s. GenST s) -> Int)
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Now we do defaulting, get alpha := Integer, and report that we can't
match Integer with (forall s. GenST s) -> Int.  That's not totally
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stupid, but perhaps a little strange.

Another potential alternative would be to suppress *all* non-insoluble
errors if there are *any* insoluble errors, anywhere, but that seems
too drastic.

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Note [Must simplify after defaulting]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We may have a deeply buried constraint
    (t:*) ~ (a:Open)
which we couldn't solve because of the kind incompatibility, and 'a' is free.
Then when we default 'a' we can solve the constraint.  And we want to do
that before starting in on type classes.  We MUST do it before reporting
errors, because it isn't an error!  Trac #7967 was due to this.

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Note [Top-level Defaulting Plan]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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We have considered two design choices for where/when to apply defaulting.
   (i) Do it in SimplCheck mode only /whenever/ you try to solve some
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       simple constraints, maybe deep inside the context of implications.
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       This used to be the case in GHC 7.4.1.
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   (ii) Do it in a tight loop at simplifyTop, once all other constraints have
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        finished. This is the current story.

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Option (i) had many disadvantages:
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   a) Firstly, it was deep inside the actual solver.
   b) Secondly, it was dependent on the context (Infer a type signature,
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      or Check a type signature, or Interactive) since we did not want
      to always start defaulting when inferring (though there is an exception to
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      this, see Note [Default while Inferring]).
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   c) It plainly did not work. Consider typecheck/should_compile/DfltProb2.hs:
          f :: Int -> Bool
          f x = const True (\y -> let w :: a -> a
                                      w a = const a (y+1)
                                  in w y)
      We will get an implication constraint (for beta the type of y):
               [untch=beta] forall a. 0 => Num beta
      which we really cannot default /while solving/ the implication, since beta is
      untouchable.

Instead our new defaulting story is to pull defaulting out of the solver loop and
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go with option (ii), implemented at SimplifyTop. Namely:
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     - First, have a go at solving the residual constraint of the whole
       program
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     - Try to approximate it with a simple constraint
     - Figure out derived defaulting equations for that simple constraint
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     - Go round the loop again if you did manage to get some equations

Now, that has to do with class defaulting. However there exists type variable /kind/
defaulting. Again this is done at the top-level and the plan is:
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     - At the top-level, once you had a go at solving the constraint, do
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       figure out /all/ the touchable unification variables of the wanted constraints.
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     - Apply defaulting to their kinds

More details in Note [DefaultTyVar].
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Note [Safe Haskell Overlapping Instances]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In Safe Haskell, we apply an extra restriction to overlapping instances. The
motive is to prevent untrusted code provided by a third-party, changing the
behavior of trusted code through type-classes. This is due to the global and
implicit nature of type-classes that can hide the source of the dictionary.

Another way to state this is: if a module M compiles without importing another
module N, changing M to import N shouldn't change the behavior of M.

Overlapping instances with type-classes can violate this principle. However,
overlapping instances aren't always unsafe. They are just unsafe when the most
selected dictionary comes from untrusted code (code compiled with -XSafe) and
overlaps instances provided by other modules.

In particular, in Safe Haskell at a call site with overlapping instances, we
apply the following rule to determine if it is a 'unsafe' overlap:

 1) Most specific instance, I1, defined in an `-XSafe` compiled module.
 2) I1 is an orphan instance or a MPTC.
 3) At least one overlapped instance, Ix, is both:
    A) from a different module than I1
    B) Ix is not marked `OVERLAPPABLE`

This is a slightly involved heuristic, but captures the situation of an
imported module N changing the behavior of existing code. For example, if
condition (2) isn't violated, then the module author M must depend either on a
type-class or type defined in N.

Secondly, when should these heuristics be enforced? We enforced them when the
type-class method call site is in a module marked `-XSafe` or `-XTrustworthy`.
This allows `-XUnsafe` modules to operate without restriction, and for Safe
Haskell inferrence to infer modules with unsafe overlaps as unsafe.

One alternative design would be to also consider if an instance was imported as
a `safe` import or not and only apply the restriction to instances imported
safely. However, since instances are global and can be imported through more
than one path, this alternative doesn't work.

Note [Safe Haskell Overlapping Instances Implementation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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How is this implemented? It's complicated! So we'll step through it all:
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 1) `InstEnv.lookupInstEnv` -- Performs instance resolution, so this is where
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    we check if a particular type-class method call is safe or unsafe. We do this
    through the return type, `ClsInstLookupResult`, where the last parameter is a
    list of instances that are unsafe to overlap. When the method call is safe,
    the list is null.
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 2) `TcInteract.matchClassInst` -- This module drives the instance resolution
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    / dictionary generation. The return type is `LookupInstResult`, which either
    says no instance matched, or one found, and if it was a safe or unsafe
    overlap.
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 3) `TcInteract.doTopReactDict` -- Takes a dictionary / class constraint and
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     tries to resolve it by calling (in part) `matchClassInst`. The resolving
     mechanism has a work list (of constraints) that it process one at a time. If
     the constraint can't be resolved, it's added to an inert set. When compiling
     an `-XSafe` or `-XTrustworthy` module, we follow this approach as we know
     compilation should fail. These are handled as normal constraint resolution
     failures from here-on (see step 6).

     Otherwise, we may be inferring safety (or using `-Wunsafe`), and
     compilation should succeed, but print warnings and/or mark the compiled module
     as `-XUnsafe`. In this case, we call `insertSafeOverlapFailureTcS` which adds
     the unsafe (but resolved!) constraint to the `inert_safehask` field of
     `InertCans`.

 4) `TcSimplify.simplifyTop`:
       * Call simpl_top, the top-level function for driving the simplifier for
         constraint resolution.

       * Once finished, call `getSafeOverlapFailures` to retrieve the
         list of overlapping instances that were successfully resolved,
         but unsafe. Remember, this is only applicable for generating warnings
         (`-Wunsafe`) or inferring a module unsafe. `-XSafe` and `-XTrustworthy`
         cause compilation failure by not resolving the unsafe constraint at all.

       * For unresolved constraints (all types), call `TcErrors.reportUnsolved`,
         while for resolved but unsafe overlapping dictionary constraints, call
         `TcErrors.warnAllUnsolved`. Both functions convert constraints into a
         warning message for the user.

       * In the case of `warnAllUnsolved` for resolved, but unsafe
         dictionary constraints, we collect the generated warning
         message (pop it) and call `TcRnMonad.recordUnsafeInfer` to
         mark the module we are compiling as unsafe, passing the
         warning message along as the reason.

 5) `TcErrors.*Unsolved` -- Generates error messages for constraints by
    actually calling `InstEnv.lookupInstEnv` again! Yes, confusing, but all we
    know is the constraint that is unresolved or unsafe. For dictionary, all we
    know is that we need a dictionary of type C, but not what instances are
    available and how they overlap. So we once again call `lookupInstEnv` to
    figure that out so we can generate a helpful error message.

 6) `TcRnMonad.recordUnsafeInfer` -- Save the unsafe result and reason in an
      IORef called `tcg_safeInfer`.

 7) `HscMain.tcRnModule'` -- Reads `tcg_safeInfer` after type-checking, calling
    `HscMain.markUnsafeInfer` (passing the reason along) when safe-inferrence
    failed.
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-}
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------------------
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simplifyAmbiguityCheck :: Type -> WantedConstraints -> TcM ()
simplifyAmbiguityCheck ty wanteds
  = do { traceTc "simplifyAmbiguityCheck {" (text "type = " <+> ppr ty $$ text "wanted = " <+> ppr wanteds)
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       ; (final_wc, _) <- runTcS $ simpl_top wanteds
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       ; traceTc "End simplifyAmbiguityCheck }" empty

       -- Normally report all errors; but with -XAllowAmbiguousTypes
       -- report only insoluble ones, since they represent genuinely
       -- inaccessible code
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       ; allow_ambiguous <- xoptM LangExt.AllowAmbiguousTypes
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       ; traceTc "reportUnsolved(ambig) {" empty
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       ; tc_lvl <- TcM.getTcLevel
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       ; unless (allow_ambiguous && not (insolubleWC tc_lvl final_wc))
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                (discardResult (reportUnsolved final_wc))
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       ; traceTc "reportUnsolved(ambig) }" empty

       ; return () }

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------------------
simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind)
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simplifyInteractive wanteds
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  = traceTc "simplifyInteractive" empty >>
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    simplifyTop wanteds
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------------------
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simplifyDefault :: ThetaType    -- Wanted; has no type variables in it
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                -> TcM ()       -- Succeeds if the constraint is soluble
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simplifyDefault theta
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  = do { traceTc "simplifyInteractive" empty
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       ; wanted <- newWanteds DefaultOrigin theta
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       ; unsolved <- simplifyWantedsTcM wanted
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       ; traceTc "reportUnsolved {" empty
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       ; reportAllUnsolved unsolved
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       ; traceTc "reportUnsolved }" empty

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       ; return () }
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------------------
tcCheckSatisfiability :: Bag EvVar -> TcM Bool
-- Return True if satisfiable, False if definitely contradictory
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tcCheckSatisfiability given_ids
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  = do { lcl_env <- TcM.getLclEnv
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       ; let given_loc = mkGivenLoc topTcLevel UnkSkol lcl_env
       ; (res, _ev_binds) <- runTcS $
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             do { traceTcS "checkSatisfiability {" (ppr given_ids)
                ; given_cts <- mkGivensWithSuperClasses given_loc (bagToList given_ids)
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                     -- See Note [Superclases and satisfiability]
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                ; insols <- solveSimpleGivens given_cts
                ; insols <- try_harder insols
                ; traceTcS "checkSatisfiability }" (ppr insols)
                ; return (isEmptyBag insols) }
       ; return res }
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 where
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    try_harder :: Cts -> TcS Cts
    -- Maybe we have to search up the superclass chain to find
    -- an unsatisfiable constraint.  Example: pmcheck/T3927b.
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    -- At the moment we try just once
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    try_harder insols
      | not (isEmptyBag insols)   -- We've found that it's definitely unsatisfiable
      = return insols             -- Hurrah -- stop now.
      | otherwise
      = do { pending_given <- getPendingScDicts
           ; new_given <- makeSuperClasses pending_given
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           ; solveSimpleGivens new_given }

{- Note [Superclases and satisfiability]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Expand superclasses before starting, because (Int ~ Bool), has
(Int ~~ Bool) as a superclass, which in turn has (Int ~N# Bool)
as a superclass, and it's the latter that is insoluble.  See
Note [The equality types story] in TysPrim.

If we fail to prove unsatisfiability we (arbitrarily) try just once to
find superclasses, using try_harder.  Reason: we might have a type
signature
   f :: F op (Implements push) => ..
where F is a type function.  This happened in Trac #3972.

We could do more than once but we'd have to have /some/ limit: in the
the recurisve case, we would go on forever in the common case where
the constraints /are/ satisfiable (Trac #10592 comment:12!).

For stratightforard situations without type functions the try_harder
step does nothing.

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***********************************************************************************
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*                                                                                 *
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*                            Inference
*                                                                                 *
***********************************************************************************
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Note [Inferring the type of a let-bound variable]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
   f x = rhs

To infer f's type we do the following:
 * Gather the constraints for the RHS with ambient level *one more than*
   the current one.  This is done by the call
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        pushLevelAndCaptureConstraints (tcMonoBinds...)
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   in TcBinds.tcPolyInfer

 * Call simplifyInfer to simplify the constraints and decide what to
   quantify over. We pass in the level used for the RHS constraints,
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   here called rhs_tclvl.
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This ensures that the implication constraint we generate, if any,
has a strictly-increased level compared to the ambient level outside
the let binding.
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-}
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simplifyInfer :: TcLevel               -- Used when generating the constraints
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              -> Bool                  -- Apply monomorphism restriction
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              -> [TcIdSigInfo]         -- Any signatures (possibly partial)
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              -> [(Name, TcTauType)]   -- Variables to be generalised,
                                       -- and their tau-types
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              -> WantedConstraints
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              -> TcM ([TcTyVar],    -- Quantify over these type variables
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                      [EvVar],      -- ... and these constraints (fully zonked)
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                      TcEvBinds)    -- ... binding these evidence variables
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simplifyInfer rhs_tclvl apply_mr sigs name_taus wanteds
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  | isEmptyWC wanteds
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  = do { gbl_tvs <- tcGetGlobalTyCoVars
       ; qtkvs <- quantify_tvs sigs gbl_tvs $
                  splitDepVarsOfTypes (map snd name_taus)
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       ; traceTc "simplifyInfer: empty WC" (ppr name_taus $$ ppr qtkvs)
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       ; return (qtkvs, [], emptyTcEvBinds) }
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  | otherwise
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  = do { traceTc "simplifyInfer {"  $ vcat
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             [ text "sigs =" <+> ppr sigs
             , text "binds =" <+> ppr name_taus
             , text "rhs_tclvl =" <+> ppr rhs_tclvl
             , text "apply_mr =" <+> ppr apply_mr
             , text "(unzonked) wanted =" <+> ppr wanteds
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             ]

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       -- First do full-blown solving
       -- NB: we must gather up all the bindings from doing
       -- this solving; hence (runTcSWithEvBinds ev_binds_var).
       -- And note that since there are nested implications,
       -- calling solveWanteds will side-effect their evidence
       -- bindings, so we can't just revert to the input
       -- constraint.
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       ; ev_binds_var <- TcM.newTcEvBinds
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       ; wanted_transformed_incl_derivs <- setTcLevel rhs_tclvl $
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           do { sig_derived <- concatMapM mkSigDerivedWanteds sigs
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                  -- the False says we don't really need to solve all Deriveds
              ; runTcSWithEvBinds False (Just ev_binds_var) $
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                solveWanteds (wanteds `addSimples` listToBag sig_derived) }
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       ; wanted_transformed_incl_derivs <- TcM.zonkWC wanted_transformed_incl_derivs
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       -- Find quant_pred_candidates, the predicates that
       -- we'll consider quantifying over
       -- NB: We do not do any defaulting when inferring a type, this can lead
       -- to less polymorphic types, see Note [Default while Inferring]
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       ; tc_lcl_env <- TcM.getLclEnv
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       ; let wanted_transformed = dropDerivedWC wanted_transformed_incl_derivs
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       ; quant_pred_candidates   -- Fully zonked
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           <- if insolubleWC rhs_tclvl wanted_transformed_incl_derivs
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              then return []   -- See Note [Quantification with errors]
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                               -- NB: must include derived errors in this test,
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                               --     hence "incl_derivs"

              else do { let quant_cand = approximateWC wanted_transformed
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                            meta_tvs   = filter isMetaTyVar (varSetElems (tyCoVarsOfCts quant_cand))

                      ; gbl_tvs <- tcGetGlobalTyCoVars
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                            -- Miminise quant_cand.  We are not interested in any evidence
                            -- produced, because we are going to simplify wanted_transformed
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                            -- again later. All we want here are the predicates over which to
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                            -- quantify.
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                            --
                            -- If any meta-tyvar unifications take place (unlikely), we'll
                            -- pick that up later.

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                      -- See Note [Promote _and_ default when inferring]
                      ; let def_tyvar tv
                              = when (not $ tv `elemVarSet` gbl_tvs) $
                                defaultTyVar tv
                      ; mapM_ def_tyvar meta_tvs
                      ; mapM_ (promoteTyVar rhs_tclvl) meta_tvs

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                      ; WC { wc_simple = simples }
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                           <- setTcLevel rhs_tclvl $
                              runTcSDeriveds       $
                              solveSimpleWanteds $ mapBag toDerivedCt quant_cand
                                -- NB: we don't want evidence, so used
                                -- Derived constraints

                      ; simples <- TcM.zonkSimples simples
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                      ; return [ ctEvPred ev | ct <- bagToList simples
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                                             , let ev = ctEvidence ct ] }
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       -- NB: quant_pred_candidates is already fully zonked
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       -- Decide what type variables and constraints to quantify
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       ; zonked_taus <- mapM (TcM.zonkTcType . snd) name_taus
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       ; let zonked_tau_tkvs = splitDepVarsOfTypes zonked_taus
       ; (qtvs, bound_theta)
           <- decideQuantification apply_mr sigs name_taus
                                   quant_pred_candidates zonked_tau_tkvs

         -- Promote any type variables that are free in the inferred type
         -- of the function:
         --    f :: forall qtvs. bound_theta => zonked_tau
         -- These variables now become free in the envt, and hence will show
         -- up whenever 'f' is called.  They may currently at rhs_tclvl, but
         -- they had better be unifiable at the outer_tclvl!
         -- Example:   envt mentions alpha[1]
         --            tau_ty = beta[2] -> beta[2]
         --            consraints = alpha ~ [beta]
         -- we don't quantify over beta (since it is fixed by envt)
         -- so we must promote it!  The inferred type is just
         --   f :: beta -> beta
       ; outer_tclvl    <- TcM.getTcLevel
       ; zonked_tau_tvs <- fold <$>
                           traverse TcM.zonkTyCoVarsAndFV zonked_tau_tkvs
              -- decideQuantification turned some meta tyvars into
              -- quantified skolems, so we have to zonk again

       ; let phi_tvs     = tyCoVarsOfTypes bound_theta
                           `unionVarSet` zonked_tau_tvs

             promote_tvs = closeOverKinds phi_tvs `delVarSetList` qtvs
       ; MASSERT2( closeOverKinds promote_tvs `subVarSet` promote_tvs
                 , ppr phi_tvs $$
                   ppr (closeOverKinds phi_tvs) $$
                   ppr promote_tvs $$
                   ppr (closeOverKinds promote_tvs) )
           -- we really don't want a type to be promoted when its kind isn't!

           -- promoteTyVar ignores coercion variables
       ; mapM_ (promoteTyVar outer_tclvl) (varSetElems promote_tvs)

           -- Emit an implication constraint for the
           -- remaining constraints from the RHS
       ; bound_theta_vars <- mapM TcM.newEvVar bound_theta
       ; let skol_info   = InferSkol [ (name, mkSigmaTy [] bound_theta ty)
                                     | (name, ty) <- name_taus ]
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                        -- Don't add the quantified variables here, because
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                        -- they are also bound in ic_skols and we want them
                        -- to be tidied uniformly
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             implic = Implic { ic_tclvl    = rhs_tclvl
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                             , ic_skols    = qtvs
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                             , ic_no_eqs   = False
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                             , ic_given    = bound_theta_vars
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                             , ic_wanted   = wanted_transformed
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                             , ic_status   = IC_Unsolved
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                             , ic_binds    = Just ev_binds_var
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                             , ic_info     = skol_info
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                             , ic_env      = tc_lcl_env }
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       ; emitImplication implic
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         -- All done!
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       ; traceTc "} simplifyInfer/produced residual implication for quantification" $
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         vcat [ text "quant_pred_candidates =" <+> ppr quant_pred_candidates
              , text "zonked_taus" <+> ppr zonked_taus
              , text "zonked_tau_tvs=" <+> ppr zonked_tau_tvs
              , text "promote_tvs=" <+> ppr promote_tvs
              , text "bound_theta =" <+> ppr bound_theta
              , text "qtvs =" <+> ppr qtvs
              , text "implic =" <+> ppr implic ]
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       ; return ( qtvs, bound_theta_vars, TcEvBinds ev_binds_var ) }
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mkSigDerivedWanteds :: TcIdSigInfo -> TcM [Ct]
-- See Note [Add deriveds for signature contexts]
mkSigDerivedWanteds (TISI { sig_bndr = PartialSig { sig_name = name }
                          , sig_theta = theta, sig_tau = tau })
 = do { let skol_info = InferSkol [(name, mkSigmaTy [] theta tau)]
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      ; loc <- getCtLocM (GivenOrigin skol_info) (Just TypeLevel)
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      ; return [ mkNonCanonical (CtDerived { ctev_pred = pred
                                           , ctev_loc = loc })
               | pred <- theta ] }
mkSigDerivedWanteds _ = return []

{- Note [Add deriveds for signature contexts]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this (Trac #11016):
  f2 :: (?x :: Int) => _
  f2 = ?x
We'll use plan InferGen because there are holes in the type.  But we want
to have the (?x :: Int) constraint floating around so that the functional
dependencies kick in.  Otherwise the occurrence of ?x on the RHS produces
constraint (?x :: alpha), and we wont unify alpha:=Int.

Solution: in simplifyInfer, just before simplifying the constraints
gathered from the RHS, add Derived constraints for the context of any
type signatures.  This is rare; if there is a type signature we'll usually
be doing CheckGen.  But it happens for signatures with holes.

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************************************************************************
*                                                                      *
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                Quantification
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*                                                                      *
************************************************************************
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Note [Deciding quantification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If the monomorphism restriction does not apply, then we quantify as follows:
  * Take the global tyvars, and "grow" them using the equality constraints
    E.g.  if x:alpha is in the environment, and alpha ~ [beta] (which can
          happen because alpha is untouchable here) then do not quantify over
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          beta, because alpha fixes beta, and beta is effectively free in
          the environment too
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    These are the mono_tvs

  * Take the free vars of the tau-type (zonked_tau_tvs) and "grow" them
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    using all the constraints.  These are tau_tvs_plus
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  * Use quantifyTyVars to quantify over (tau_tvs_plus - mono_tvs), being
    careful to close over kinds, and to skolemise the quantified tyvars.
    (This actually unifies each quantifies meta-tyvar with a fresh skolem.)
    Result is qtvs.
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  * Filter the constraints using pickQuantifiablePreds and the qtvs.
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    We have to zonk the constraints first, so they "see" the freshly
    created skolems.
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If the MR does apply, mono_tvs includes all the constrained tyvars --
including all covars -- and the quantified constraints are empty/insoluble.

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-}
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decideQuantification
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  :: Bool                  -- try the MR restriction?
  -> [TcIdSigInfo]
  -> [(Name, TcTauType)]   -- variables to be generalised (for errors only)
  -> [PredType]            -- candidate theta
  -> Pair TcTyCoVarSet     -- dependent (kind) variables & type variables
  -> TcM ( [TcTyVar]       -- Quantify over these (skolems)
         , [PredType] )    -- and this context (fully zonked)
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-- See Note [Deciding quantification]
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decideQuantification apply_mr sigs name_taus constraints
                     zonked_pair@(Pair zonked_tau_kvs zonked_tau_tvs)
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  | apply_mr     -- Apply the Monomorphism restriction
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  = do { gbl_tvs <- tcGetGlobalTyCoVars
       ; let constrained_tvs = tyCoVarsOfTypes constraints `unionVarSet`
                               filterVarSet isCoVar zonked_tkvs
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             mono_tvs = gbl_tvs `unionVarSet` constrained_tvs
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       ; qtvs <- quantify_tvs sigs mono_tvs zonked_pair
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           -- Warn about the monomorphism restriction
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       ; warn_mono <- woptM Opt_WarnMonomorphism
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       ; let mr_bites = constrained_tvs `intersectsVarSet` zonked_tkvs
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       ; warnTc (warn_mono && mr_bites) $
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         hang (text "The Monomorphism Restriction applies to the binding"
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               <> plural bndrs <+> text "for" <+> pp_bndrs)
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             2 (text "Consider giving a type signature for"
                <+> if isSingleton bndrs then pp_bndrs
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                                         else text "these binders")
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       -- All done
       ; traceTc "decideQuantification 1" (vcat [ppr constraints, ppr gbl_tvs, ppr mono_tvs
                                                , ppr qtvs, ppr mr_bites])
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       ; return (qtvs, []) }
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  | otherwise
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  = do { gbl_tvs <- tcGetGlobalTyCoVars
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       ; let mono_tvs     = growThetaTyVars equality_constraints gbl_tvs
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             tau_tvs_plus = growThetaTyVars constraints zonked_tau_tvs
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       ; qtvs <- quantify_tvs sigs mono_tvs (Pair zonked_tau_kvs tau_tvs_plus)
          -- We don't grow the kvs, as there's no real need to. Recall
          -- that quantifyTyVars uses the separation between kvs and tvs
          -- only for defaulting, and we don't want (ever) to default a tv
          -- to *. So, don't grow the kvs.

       ; constraints <- TcM.zonkTcTypes constraints
                 -- quantiyTyVars turned some meta tyvars into
                 -- quantified skolems, so we have to zonk again

       ; let theta     = pickQuantifiablePreds (mkVarSet qtvs) constraints
             min_theta = mkMinimalBySCs theta
               -- See Note [Minimize by Superclasses]

       ; traceTc "decideQuantification 2"
           (vcat [ text "constraints:"  <+> ppr constraints
                 , text "gbl_tvs:"      <+> ppr gbl_tvs
                 , text "mono_tvs:"     <+> ppr mono_tvs
                 , text "zonked_kvs:"   <+> ppr zonked_tau_kvs
                 , text "tau_tvs_plus:" <+> ppr tau_tvs_plus
                 , text "qtvs:"         <+> ppr qtvs
                 , text "min_theta:"    <+> ppr min_theta ])
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       ; return (qtvs, min_theta) }
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  where
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    zonked_tkvs = zonked_tau_kvs `unionVarSet` zonked_tau_tvs
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    bndrs    = map fst name_taus
    pp_bndrs = pprWithCommas (quotes . ppr) bndrs
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    equality_constraints = filter isEqPred constraints

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quantify_tvs :: [TcIdSigInfo]
             -> TcTyVarSet   -- the monomorphic tvs
             -> Pair TcTyVarSet   -- kvs, tvs to quantify
             -> TcM [TcTyVar]
-- See Note [Which type variables to quantify]
quantify_tvs sigs mono_tvs (Pair tau_kvs tau_tvs)
  = quantifyTyVars (mono_tvs `delVarSetList` sig_qtvs)
                   (Pair tau_kvs
                         (tau_tvs `extendVarSetList` sig_qtvs
                                  `extendVarSetList` sig_wcs))
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                   -- NB: quantifyTyVars zonks its arguments
  where
    sig_qtvs = [ skol | sig <- sigs, (_, skol) <- sig_skols sig ]
    sig_wcs  = [ wc   | TISI { sig_bndr = PartialSig { sig_wcs = wcs } } <- sigs
                      , (_, wc) <- wcs ]
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------------------
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growThetaTyVars :: ThetaType -> TyCoVarSet -> TyVarSet
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-- See Note [Growing the tau-tvs using constraints]
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-- NB: only returns tyvars, never covars
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growThetaTyVars theta tvs
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  | null theta = tvs_only
  | otherwise  = filterVarSet isTyVar $
                 transCloVarSet mk_next seed_tvs
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  where
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    tvs_only = filterVarSet isTyVar tvs
    seed_tvs = tvs `unionVarSet` tyCoVarsOfTypes ips
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    (ips, non_ips) = partition isIPPred theta
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                         -- See Note [Inheriting implicit parameters] in TcType
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    mk_next :: VarSet -> VarSet -- Maps current set to newly-grown ones
    mk_next so_far = foldr (grow_one so_far) emptyVarSet non_ips
    grow_one so_far pred tvs
       | pred_tvs `intersectsVarSet` so_far = tvs `unionVarSet` pred_tvs
       | otherwise                          = tvs
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       where
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         pred_tvs = tyCoVarsOfType pred
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{- Note [Which type variables to quantify]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When choosing type variables to quantify, the basic plan is to
quantify over all type variables that are
 * free in the tau_tvs, and
 * not forced to be monomorphic (mono_tvs),
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   for example by being free in the environment.
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However, for a pattern binding, or with wildcards, we might
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be doing inference *in the presence of a type signature*.
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Mostly, if there is a signature we use CheckGen, not InferGen,
but with pattern bindings or wildcards we might do InferGen
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and still have a type signature.  For example:
   f :: _ -> a
   f x = ...
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or
   g :: (Eq _a) => _b -> _b
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or
   p :: a -> a
   (p,q) = e
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In all these cases we use plan InferGen, and hence call simplifyInfer.
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But those 'a' variables are skolems, and we should be sure to quantify
over them, regardless of the monomorphism restriction etc.  If we
don't, when reporting a type error we panic when we find that a
skolem isn't bound by any enclosing implication.

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Moreover we must quantify over all wildcards that are not free in
the environment.  In the case of 'g' for example, silly though it is,
we want to get the inferred type
   g :: forall t. Eq t => Int -> Int
and then report ambiguity, rather than *not* quantifying over 't'
and getting some much more mysterious error later.  A similar case
is
  h :: F _a -> Int

That's why we pass sigs to simplifyInfer, and make sure (in
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quantify_tvs) that we do quantify over them.  Trac #10615 is
a case in point.

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Note [Quantifying over equality constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Should we quantify over an equality constraint (s ~ t)?  In general, we don't.
Doing so may simply postpone a type error from the function definition site to
its call site.  (At worst, imagine (Int ~ Bool)).

However, consider this
         forall a. (F [a] ~ Int) => blah
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Should we quantify over the (F [a] ~ Int)?  Perhaps yes, because at the call
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site we will know 'a', and perhaps we have instance  F [Bool] = Int.
So we *do* quantify over a type-family equality where the arguments mention
the quantified variables.

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Note [Growing the tau-tvs using constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(growThetaTyVars insts tvs) is the result of extending the set
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    of tyvars, tvs, using all conceivable links from pred
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E.g. tvs = {a}, preds = {H [a] b, K (b,Int) c, Eq e}
Then growThetaTyVars preds tvs = {a,b,c}

Notice that
   growThetaTyVars is conservative       if v might be fixed by vs
                                         => v `elem` grow(vs,C)

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Note [Quantification with errors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we find that the RHS of the definition has some absolutely-insoluble
constraints, we abandon all attempts to find a context to quantify
over, and instead make the function fully-polymorphic in whatever
type we have found.  For two reasons
  a) Minimise downstream errors
  b) Avoid spurious errors from this function
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But NB that we must include *derived* errors in the check. Example:
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    (a::*) ~ Int#
We get an insoluble derived error *~#, and we don't want to discard
it before doing the isInsolubleWC test!  (Trac #8262)
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Note [Default while Inferring]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Our current plan is that defaulting only happens at simplifyTop and
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not simplifyInfer.  This may lead to some insoluble deferred constraints.
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Example:

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instance D g => C g Int b
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constraint inferred = (forall b. 0 => C gamma alpha b) /\ Num alpha
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type inferred       = gamma -> gamma
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Now, if we try to default (alpha := Int) we will be able to refine the implication to
  (forall b. 0 => C gamma Int b)
which can then be simplified further to
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  (forall b. 0 => D gamma)
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Finally, we /can/ approximate this implication with (D gamma) and infer the quantified
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type:  forall g. D g => g -> g

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Instead what will currently happen is that we will get a quantified type
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(forall g. g -> g) and an implication:
       forall g. 0 => (forall b. 0 => C g alpha b) /\ Num alpha

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Which, even if the simplifyTop defaults (alpha := Int) we will still be left with an
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unsolvable implication:
       forall g. 0 => (forall b. 0 => D g)

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The concrete example would be:
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       h :: C g a s => g -> a -> ST s a
       f (x::gamma) = (\_ -> x) (runST (h x (undefined::alpha)) + 1)

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But it is quite tedious to do defaulting and resolve the implication constraints, and
we have not observed code breaking because of the lack of defaulting in inference, so
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we don't do it for now.



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Note [Minimize by Superclasses]
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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When we quantify over a constraint, in simplifyInfer we need to
quantify over a constraint that is minimal in some sense: For
instance, if the final wanted constraint is (Eq alpha, Ord alpha),
we'd like to quantify over Ord alpha, because we can just get Eq alpha
from superclass selection from Ord alpha. This minimization is what
mkMinimalBySCs does. Then, simplifyInfer uses the minimal constraint
to check the original wanted.
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Note [Avoid unnecessary constraint simplification]
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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    -------- NB NB NB (Jun 12) -------------
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    This note not longer applies; see the notes with Trac #4361.
    But I'm leaving it in here so we remember the issue.)
    ----------------------------------------
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When inferring the type of a let-binding, with simplifyInfer,
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try to avoid unnecessarily simplifying class constraints.
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Doing so aids sharing, but it also helps with delicate