Role annotation should never be ambiguous

[ezyang@hs01 ghc-backpack]$ cat A.hs
{-# LANGUAGE RoleAnnotations #-}
module A where
data Maybe a = AF
type role Maybe representational
[ezyang@hs01 ghc-backpack]$ ghc -c A.hs

A.hs:4:11:
    Ambiguous occurrence ‘Maybe’
    It could refer to either ‘A.Maybe’, defined at A.hs:3:1
                          or ‘Prelude.Maybe’,
                             imported from ‘Prelude’ at A.hs:2:8
                             (and originally defined in ‘GHC.Base’)

This is silly because roles can only ever be defined with the declaration, so only one reference makes sense.

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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