better type inference for lambdas

With the following code in a file:

{-# LANGUAGE TypeFamilies #-}
class Test a where test :: a
instance (a ~ Int, b ~ Int) => Test (a -> b) where test = id

We can tell ghci that test is a function (and nothing more) in two different ways:

*Main> :t test `asTypeOf` (undefined :: a -> b)
test `asTypeOf` (undefined :: a -> b) :: Int -> Int
*Main> :t \x -> test x
\x -> test x :: Test (t1 -> t) => t1 -> t

The type inferred for \x -> test x is very disappointing in comparison! A similar example can be cooked up with MPTCs and functional dependencies; see also http://lpaste.net/91534 for some (still quite abstract, of course) motivation.

Trac metadata

Trac field	Value
Version	7.6.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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