Implication constraint priority breaks default class implementations

Hello,

Not 100% sure that this is a bug, but I've done some investigating (with a lot of help from Csongor Kiss) and thought it was, at the very least, behaviour worth clarifying. The following code...

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE UndecidableInstances #-}
module Test where

class D a where
  f :: a -> String
  g :: a -> String
  g = f

class C a

instance (forall a. C a => D a) => D x where
  f _ = "uh oh"

1. .. produces the error:

    • Could not deduce (C x) arising from a use of ‘Test.$dmg’
      from the context: forall a. C a => D a
        bound by the instance declaration at Test.hs:19:10-38
      Possible fix: add (C x) to the context of the instance declaration
    • In the expression: Test.$dmg @(x)
      In an equation for ‘g’: g = Test.$dmg @(x)
      In the instance declaration for ‘D x’
   |
19 | instance (forall a. C a => D a) => D x where
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

It appears that the problem here is with the default implementation for g. Namely, when f is called, two matching instances are found:

• forall a. C a => D a
• (forall a. C a => D a) => D x

The issue, as far as we can tell, is that the first instance is chosen (and then the constraint check fails). I'm currently working around this by introducing a newtype into the head of the quantified constraint†, but I thought it best to check whether this is a bug or, indeed, the expected behaviour in this situation.

Let me know if I've missed anything from this ticket - it's my first one!

Thanks, Tom

† An example of this can be found at https://github.com/i-am-tom/learn-me-a-haskell/blob/dbf2a22c5abb78ab91124dcf1e0e7ecd3d88831d/src/Bag/QuantifiedInstances.hs#L92-L94

Trac metadata

Trac field	Value
Version	8.6.1-beta1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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