PartialTypeSignatures suggests a redundant constraint with constraint families

When I say

{-# LANGUAGE ConstraintKinds, TypeFamilies #-}

module Bug where

type family ShowSyn a where ShowSyn a = Show a

foo :: (ShowSyn a, _) => a -> String
foo x = show x

I get

Bug.hs:7:20: error:
    Found constraint wildcard ‘_’ standing for ‘Show a’
    To use the inferred type, enable PartialTypeSignatures
    In the type signature:
      foo :: (ShowSyn a, _) => a -> String

But if I fill in my wildcard with Show a, I'll rightly get a redundant constraint. Just a vanilla type synonym works there, but the type family causes trouble.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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