Silly error message suggesting LiberalTypeSynonyms

The following code (rightly) does not compile:

{-# LANGUAGE RankNTypes, TypeFamilies, LiberalTypeSynonyms #-}

type PolyId = (forall a. a -> a)

type family F a

foo :: F PolyId
foo = undefined

The error message is

    Illegal polymorphic or qualified type: PolyId
    Perhaps you intended to use -XLiberalTypeSynonyms
    In the type signature for `foo': foo :: F PolyId

The code doesn't compile because we can't apply a type family to a polytype. The first line of the error message says this well enough, though more information might be helpful here. However, the second line of the error message is silly: I've turned on LiberalTypeSynonyms!

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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