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| author | Simon Peyton Jones <simonpj@microsoft.com> | 2020-09-08 19:25:11 +0100 |
|---|---|---|
| committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2020-09-15 15:19:44 -0400 |
| commit | c7182a5c67fe8b5bd256cb8eb805562636853ea2 (patch) | |
| tree | 6e9f04053559df514b8deab624bb9491fae27f27 /compiler/GHC/Tc/Instance | |
| parent | 8610bcbeb11b898f85f228b755fa8421b5ae3e34 (diff) | |
| download | haskell-c7182a5c67fe8b5bd256cb8eb805562636853ea2.tar.gz | |
Care with implicit-parameter superclasses
Two bugs, #18627 and #18649, had the same cause: we were not
account for the fact that a constaint tuple might hide an implicit
parameter.
The solution is not hard: look for implicit parameters in
superclasses. See Note [Local implicit parameters] in
GHC.Core.Predicate.
Then we use this new function in two places
* The "short-cut solver" in GHC.Tc.Solver.Interact.shortCutSolver
which simply didn't handle implicit parameters properly at all.
This fixes #18627
* The specialiser, which should not specialise on implicit parameters
This fixes #18649
There are some lingering worries (see Note [Local implicit
parameters]) but things are much better.
Diffstat (limited to 'compiler/GHC/Tc/Instance')
| -rw-r--r-- | compiler/GHC/Tc/Instance/Class.hs | 7 |
1 files changed, 3 insertions, 4 deletions
diff --git a/compiler/GHC/Tc/Instance/Class.hs b/compiler/GHC/Tc/Instance/Class.hs index 278b63f03c..b66ad2d447 100644 --- a/compiler/GHC/Tc/Instance/Class.hs +++ b/compiler/GHC/Tc/Instance/Class.hs @@ -388,10 +388,9 @@ makeLitDict clas ty et | Just (_, co_dict) <- tcInstNewTyCon_maybe (classTyCon clas) [ty] -- co_dict :: KnownNat n ~ SNat n , [ meth ] <- classMethods clas - , Just tcRep <- tyConAppTyCon_maybe -- SNat - $ funResultTy -- SNat n - $ dropForAlls -- KnownNat n => SNat n - $ idType meth -- forall n. KnownNat n => SNat n + , Just tcRep <- tyConAppTyCon_maybe (classMethodTy meth) + -- If the method type is forall n. KnownNat n => SNat n + -- then tcRep is SNat , Just (_, co_rep) <- tcInstNewTyCon_maybe tcRep [ty] -- SNat n ~ Integer , let ev_tm = mkEvCast et (mkTcSymCo (mkTcTransCo co_dict co_rep)) |