Order matters when type-checking

When I say

{-# LANGUAGE RankNTypes #-}

module Bug where

foo True  = (\x -> x) :: (forall a. a -> a) -> forall b. b -> b
foo False = \y -> y

the module compiles. But when I say

{-# LANGUAGE RankNTypes #-}

module Bug where

foo False = \y -> y
foo True  = (\x -> x) :: (forall a. a -> a) -> forall b. b -> b

it doesn't, failing with

Bug.hs:6:13:
    Couldn't match type ‘b0 -> b0’ with ‘forall a. a -> a’
    Expected type: (forall a. a -> a) -> forall a. a -> a
      Actual type: (forall a. a -> a) -> b0 -> b0
    In the expression:
        (\ x -> x) :: (forall a. a -> a) -> forall b. b -> b
    In an equation for ‘foo’:
        foo True = (\ x -> x) :: (forall a. a -> a) -> forall b. b -> b

I believe this behavior stems from the special case in tcMonoBinds, for non-recursive functions without a type signature. I believe the bug would be fixed if that function also checks to make sure that there is precisely one clause to the function. Do you agree?

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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