Commit 3098d214 authored by's avatar

Fix pre-subsumption and pre-matching

The pre-subsuption and pre-matching functions should NEVER make bogus
bindings of type variables, although they are free to bale out and make
too few bindings.

I hadn't been thiking carefully enough about this, and there were two
separate bugs.  

- Firstly, in pre-subsumption we must ignore the 'theta'
  part of any overloaded type.  

- Second, in pre-matching, we must return the empty subustition 
  on a mis-match, rather than returning the substitution so far.

This bug showed up when compiling Data.Generics.Schemes.hs, and is
imortalised in test tc206
parent d2b27dcd
......@@ -66,7 +66,7 @@ import TysWiredIn ( listTyCon )
import Id ( Id, mkSysLocal )
import Var ( Var, varName, tyVarKind, isTcTyVar, tcTyVarDetails )
import VarSet ( emptyVarSet, mkVarSet, unitVarSet, unionVarSet, elemVarSet, varSetElems,
extendVarSet, intersectsVarSet )
extendVarSet, intersectsVarSet, extendVarSetList )
import VarEnv
import Name ( Name, isSystemName )
import ErrUtils ( Message )
......@@ -356,7 +356,9 @@ preSubType :: [TcTyVar] -- Quantified type variables
-- instantiate it to a monotype (non-boxy) type variable
preSubType qtvs btvs qty expected_ty
= mapM inst_tv qtvs
= do { tys <- mapM inst_tv qtvs
; traceTc (text "preSubType" <+> (ppr qtvs $$ ppr btvs $$ ppr qty $$ ppr expected_ty $$ ppr pre_subst $$ ppr tys))
; return tys }
pre_subst = boxySubMatchType (mkVarSet qtvs) qty expected_ty
inst_tv tv
......@@ -386,9 +388,17 @@ boxySubMatchType tmpl_tvs tmpl_ty boxy_ty
-- in the template. Both look like a TyVarTy.
-- See Note [Sub-match] below
go (ForAllTy tv t_ty) b_tvs b_ty = go t_ty b_tvs b_ty -- Rule S-SPEC
go t_ty b_tvs (ForAllTy tv b_ty) = go t_ty b_tvs' b_ty -- Rule S-SKOL
where b_tvs' = extendVarSet b_tvs tv
go t_ty b_tvs b_ty
| isSigmaTy t_ty, (tvs, _, t_tau) <- tcSplitSigmaTy t_ty
= go t_tau b_tvs b_ty -- Rule S-SPEC
| isSigmaTy b_ty, (tvs, _, b_tau) <- tcSplitSigmaTy b_ty
= go t_ty (extendVarSetList b_tvs tvs) b_ty -- Rule S-SKOL
-- NB: it's *important* to discard the theta part. Otherwise
-- consider (forall a. Eq a => a -> b) ~<~ (Int -> Int -> Bool)
-- and end up with a completely bogus binding (b |-> Bool), by lining
-- up the (Eq a) with the Int, whereas it should be (b |-> (Int->Bool)).
-- This pre-subsumption stuff can return too few bindings, but it
-- must *never* return bogus info.
go (FunTy arg1 res1) b_tvs (FunTy arg2 res2) -- Rule S-FUN
= boxy_match tmpl_tvs arg1 b_tvs arg2 (go res1 b_tvs res2)
......@@ -490,12 +500,20 @@ boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
, not (intersectsVarSet boxy_tvs (tyVarsOfType orig_boxy_ty))
, typeKind b_ty `isSubKind` tyVarKind tv -- See Note [Matching kinds]
= extendTvSubst subst tv boxy_ty'
| otherwise
= subst -- Ignore others
boxy_ty' = case lookupTyVar subst tv of
Nothing -> orig_boxy_ty
Just ty -> ty `boxyLub` orig_boxy_ty
go _ _ = subst -- Always safe
go _ _ = emptyTvSubst -- It's important to *fail* by returning the empty substitution
-- Example: Tree a ~ Maybe Int
-- We do not want to bind (a |-> Int) in pre-matching, because that can give very
-- misleading error messages. An even more confusing case is
-- a -> b ~ Maybe Int
-- Then we do not want to bind (b |-> Int)! It's always safe to discard bindings
-- from this pre-matching phase.
go_s tys1 tys2 = boxy_match_s tmpl_tvs tys1 boxy_tvs tys2 subst
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