Type checker does not use functional dependency to avoid ambiguity

Compiling the following module gives an error

module X where

class C a | -> a
instance C Int

unC :: (C a) => a -> Int
unC i = undefined

test :: Int
test = unC undefined

Error message:

X.hs:13:7:
    Ambiguous type variable `a' in the constraint:
      `C a' arising from a use of `unC' at X.hs:13:7-19
    Probable fix: add a type signature that fixes these type variable(s)

But that is just plain wrong. The functional dependency in the definition of C forces a to be Int. No other type is possible. So what's ambiguous about that?

Trac metadata

Trac field	Value
Version	6.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	lennart@augustsson.net
Operating system	MacOS X
Architecture	x86