UndecidableInstances validity checking is wrong in the presence of QuantifiedConstraints

Consider this program:

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE QuantifiedConstraints #-}
module Bug where

import Data.Kind

data ECC :: Constraint -> Type -> Type

class Y a
class Z a

instance  c         => Y (ECC c a)
instance (c => Z a) => Z (ECC c a)

I would expect both of these instances to work. But while GHC accepts the Y instance, it rejects the Z instance:

$ ~/Software/ghc5/inplace/bin/ghc-stage2 Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:15:10: error:
    • Variable ‘c’ occurs more often
        in the constraint ‘c’ than in the instance head
      (Use UndecidableInstances to permit this)
    • In the instance declaration for ‘Z (ECC c a)’
   |
15 | instance (c => Z a) => Z (ECC c a)
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^

That error message seems completely bogus to me, since c appears once in both the context and instance head in both instances.

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed milestone to %8.6.1

changed weight to 5

added Tbug Trac import typechecker labels

GHC is following the proposal (Section 3.7). Specifically, in the quantified predicate c => Z a, c does occur more often in the constraint to the left of the arrow, namely c, than in the head of the quantified constraint Z a.

The error message is poor. How about this?

    • Variable ‘c’ occurs more often
        in the constraint ‘c’ 
        than in the instance head `Z a'
      (Use UndecidableInstances to permit this)
    • In the quantified constraint `c => Z a`
    • In the instance declaration for ‘Z (ECC c a)’

Ah, gotcha! That error message does sound like an improvement, yes.

Trac metadata

Trac field	Value
Differential revisions	- → D4819

mentioned in commit 91822e4e

changed milestone to %8.8.1

These won't be addressed in GHC 8.6.

closed

changed milestone to %8.6.1

Actually, this is //in// GHC 8.6, so this is fixed.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed
Test case	- → quantified-constraints/T15231

added 1 deleted label

added Pnormal label