Commit 50249a9f authored by Simon Peyton Jones's avatar Simon Peyton Jones Committed by Marge Bot

Use transSuperClasses in TcErrors

Code in TcErrors was recursively using immSuperClasses,
which loops in the presence of UndecidableSuperClasses.

Better to use transSuperClasses instead, which has a loop-breaker
mechanism built in.

Fixes issue #16414.
parent 5165378d
......@@ -2577,15 +2577,15 @@ mk_dict_err ctxt@(CEC {cec_encl = implics}) (ct, (matches, unifiers, unsafe_over
2 (sep [ text "bound by" <+> ppr skol_info
, text "at" <+>
ppr (tcl_loc (implicLclEnv implic)) ])
where ev_vars_matching = filter ev_var_matches (map evVarPred evvars)
ev_var_matches ty = case getClassPredTys_maybe ty of
Just (clas', tys')
| clas' == clas
, Just _ <- tcMatchTys tys tys'
-> True
| otherwise
-> any ev_var_matches (immSuperClasses clas' tys')
Nothing -> False
where ev_vars_matching = [ pred
| ev_var <- evvars
, let pred = evVarPred ev_var
, any can_match (pred : transSuperClasses pred) ]
can_match pred
= case getClassPredTys_maybe pred of
Just (clas', tys') -> clas' == clas
&& isJust (tcMatchTys tys tys')
Nothing -> False
-- Overlap error because of Safe Haskell (first
-- match should be the most specific match)
......@@ -2716,7 +2716,7 @@ the alleged "provided" constraints, Show a.
So we suppress that Implication in discardProvCtxtGivens. It's
painfully ad-hoc but the truth is that adding it to the "required"
constraints would work. Suprressing it solves two problems. First,
constraints would work. Suppressing it solves two problems. First,
we never tell the user that we could not deduce a "provided"
constraint from the "required" context. Second, we never give a
possible fix that suggests to add a "provided" constraint to the
......@@ -2011,8 +2011,25 @@ isInsolubleOccursCheck eq_rel tv ty
When we expand superclasses, we use the following algorithm:
expand( so_far, pred ) returns the transitive superclasses of pred,
not including pred itself
transSuperClasses( C tys ) returns the transitive superclasses
of (C tys), not including C itself
For example
class C a b => D a b
class D b a => C a b
transSuperClasses( Ord ty ) = [Eq ty]
transSuperClasses( C ta tb ) = [D tb ta, C tb ta]
Notice that in the recursive-superclass case we include C again at
the end of the chain. One could exclude C in this case, but
the code is more awkward and there seems no good reason to do so.
(However C.f. TcCanonical.mk_strict_superclasses, which /does/
appear to do so.)
The algorithm is expand( so_far, pred ):
1. If pred is not a class constraint, return empty set
Otherwise pred = C ts
2. If C is in so_far, return empty set (breaks loops)
......@@ -2024,6 +2041,8 @@ Notice that
* With normal Haskell-98 classes, the loop-detector will never bite,
so we'll get all the superclasses.
* We need the loop-breaker in case we have UndecidableSuperClasses on
* Since there is only a finite number of distinct classes, expansion
must terminate.
{-# LANGUAGE FlexibleContexts, FlexibleInstances, AllowAmbiguousTypes #-}
{-# LANGUAGE UndecidableSuperClasses #-}
module T16414 where
data I = I
class All2 x => All x
class All x => All2 x
class AllZip2 f
instance AllZip2 f
f1 :: (All x, AllZip2 I) => x -> ()
f1 = f2
f2 :: AllZip2 f => x -> ()
f2 _ = ()
T16414.hs:14:6: error:
• Overlapping instances for AllZip2 f0 arising from a use of ‘f2’
Matching givens (or their superclasses):
AllZip2 I
bound by the type signature for:
f1 :: forall x. (All x, AllZip2 I) => x -> ()
at T16414.hs:13:1-35
Matching instances:
instance AllZip2 f -- Defined at T16414.hs:11:10
(The choice depends on the instantiation of ‘f0’)
• In the expression: f2
In an equation for ‘f1’: f1 = f2
......@@ -510,3 +510,4 @@ test('T16059e', [extra_files(['T16059b.hs'])], multimod_compile_fail,
['T16059e', '-v0'])
test('T16255', normal, compile_fail, [''])
test('T16204c', normal, compile_fail, [''])
test('T16414', normal, compile_fail, [''])
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