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Diffstat (limited to 'compiler/GHC/Core/RoughMap.hs')
| -rw-r--r-- | compiler/GHC/Core/RoughMap.hs | 6 |
1 files changed, 5 insertions, 1 deletions
diff --git a/compiler/GHC/Core/RoughMap.hs b/compiler/GHC/Core/RoughMap.hs index 7107198cc6..0fa868c748 100644 --- a/compiler/GHC/Core/RoughMap.hs +++ b/compiler/GHC/Core/RoughMap.hs @@ -320,7 +320,11 @@ roughMatchTcsLookup tys = map typeToRoughMatchLookupTc tys typeToRoughMatchLookupTc :: Type -> RoughMatchLookupTc typeToRoughMatchLookupTc ty - | Just (ty', _) <- splitCastTy_maybe ty + -- Expand synonyms first, as explained in Note [Rough matching in class and family instances]. + -- Failing to do so led to #22985. + | Just ty' <- coreView ty + = typeToRoughMatchLookupTc ty' + | CastTy ty' _ <- ty = typeToRoughMatchLookupTc ty' | otherwise = case splitAppTys ty of |