Possible error message improvement when -XRankNTypes is missing

I'm forwarding wishlist bug #499217 from Debian's BTS.

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If I ask ghc to interpret a higher-rank function type, I get this error:

Prelude> :t undefined :: ((forall a . a -> a) -> a -> a)

<interactive>:1:24:
    Illegal operator `.' in type `forall a . (a -> a)'
      (Use -XTypeOperators to allow operators in types)

If I turn on -XExistentialQuantification, I get this one instead:

Prelude> :t undefined :: ((forall a . a -> a) -> a -> a)

<interactive>:1:0:
    Illegal polymorphic or qualified type: forall a. a -> a
    In an expression type signature:
      (forall a. a -> a) -> a -> a

It might not be possible in the first case (where forall isn't a keyword), but in the second case it would be nice if ghc could detect my attempt to enter a higher-rank type and suggest the appropriate extension. (-XRankNTypes)

Trac metadata

Trac field	Value
Version	6.8.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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Operating system	Unknown
Architecture	Unknown