The core type-matcher Unify.match was previouly using tcView to expand
types, because it must treat newtypes as distinct from their representation.
But that meant that it also treated the PredType {C Int} as distinct from
its representation type (:TC Int). And that in turn was causing a rule
not to fire, because the argument types didn't match up.
For this to happen we need to get a situation where we have
a = :DC blah blah -- Dictionary
....(f a).....
Now a has type (:TC Int), bu the RULE for f expects an argument
of type {C Int}. Roman found that just this was happening.
-- in-scope set of the RnEnv2
-> Type -> Type -- Template and target respectively
-> Maybe TvSubstEnv
-- in-scope set of the RnEnv2
-> Type -> Type -- Template and target respectively
-> Maybe TvSubstEnv
--- This matcher works on source types; that is,
--- it respects NewTypes and PredType
+-- This matcher works on core types; that is, it ignores PredTypes
+-- Watch out if newtypes become transparent agin!
+-- this matcher must respect newtypes
-match menv subst ty1 ty2 | Just ty1' <- tcView ty1 = match menv subst ty1' ty2
- | Just ty2' <- tcView ty2 = match menv subst ty1 ty2'
+match menv subst ty1 ty2 | Just ty1' <- coreView ty1 = match menv subst ty1' ty2
+ | Just ty2' <- coreView ty2 = match menv subst ty1 ty2'
match menv subst (TyVarTy tv1) ty2
| tv1' `elemVarSet` me_tmpls menv
match menv subst (TyVarTy tv1) ty2
| tv1' `elemVarSet` me_tmpls menv