Failure to construct superclasses in instance

Consider

{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, ConstraintKinds #-}

module Foo where

class Eq a => C a b

type X a = (Eq a, Show a)

instance X a => C a [b]

HEAD gives

Foo.hs:8:10:
    Could not deduce (Eq a)
      arising from the superclasses of an instance declaration
    from the context: X a
      bound by the instance declaration at Foo.hs:8:10-23
    Possible fix: add (Eq a) to the context of the instance declaration
    In the instance declaration for ‘C a [b]’

This is fall-out from refactoring the silent-superclass story. Patch coming.

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture