Type checker error with alias

[jvoigtlaender]

On http://elm-lang.org/try, the following program:

type alias F a = a

f : F a -> a
f = identity

gives this error:

The type annotation for `f` does not match its definition.

 4| f : F a -> a
        ^^^^^^^^

The type annotation is saying:

     F b -> b 

But I am inferring that the definition has this type:

     F b -> b

[evancz]

Centralizing into #1373, will track from there.

[evancz]

This goes through fine in my development build!