Overly specific instances may not be resolved

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
class A x
class A x => B x
class A x => C x
instance B x => C x

produces

BrokenSuper.hs:8:10: error:
    • Could not deduce (A x)
        arising from the superclasses of an instance declaration
      from the context: B x
        bound by the instance declaration at BrokenSuper.hs:8:10-19
      Possible fix: add (A x) to the context of the instance declaration
    • In the instance declaration for ‘C x’
  |
8 | instance B x => C x

This was noticed in this SO question and reduced by Daniel Wagner.

Trac metadata

Trac field	Value
Version	8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	goldfire
Operating system
Architecture