Type error when annotating a let binding with a forall type

I'm not sure if this is a bug or not, but it's certainly counterintuitive to me.

Why does this example fail to type check?

Prelude> let (x :: forall a. a -> a) = id in x 3

<interactive>:0:31:
    Couldn't match expected type `forall a. a -> a'
                with actual type `a0 -> a0'
    In the expression: id
    In a pattern binding: (x :: forall a. a -> a) = id

I have RankNTypes and ScopedTypeVariables enabled.

I asked this question on StackOverflow: http://stackoverflow.com/questions/8190431/type-error-when-ascribing-a-valid-forall-type-to-a-let-bound-variable

User ibid's theory there was this:

"In a let binding let x = id in ..., what happens is that id's type forall a. a->a gets instantiated into a monotype, say a0 -> a0, which is then assigned as x's type and is then generalized as forall a0. a0 -> a0. If, as I think, the pattern type signature is checked before generalization, it's essentially asking the compiler to verify that the monotype a0 -> a0 is more general than the polytype forall a. a -> a, which it isn't."

Trac metadata

Trac field	Value
Version	7.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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