Rank 1 type signature still requires RankNTypes

When trying to figure out which type variable names are *actually* bound in ScopedTypeVariables, I tried floating foralls into the covariant argument of the function type. Essentially, I ran into the problem that programs like the following are rejected:

{-# LANGUAGE ExplicitForAll #-}
tuple :: forall a. a -> (forall b. b -> (a, b))
tuple = (,)

The message is as follows:

Main.hs:2:10:
    Illegal polymorphic or qualified type: forall b. b -> (a, b)
    Perhaps you intended to use -XRankNTypes or -XRank2Types
    In the type signature for `tuple':
      tuple :: forall a. a -> (forall b. b -> (a, b))

As far as I know, the rank of a type is defined by how deep quantifiers are nested in contravariant parts of that type. Or something like that. Also, as far as I know, forall a. a -> (forall b. b -> (a, b)) is equivalent to forall a b. a -> b -> (a, b), and more importantly, both are rank-1 polymorphic. There should be no need to use extensions that allow higher-ranked polymorphism.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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