Type class overlapping instances check doesn't understand type equality

Here is a sample program which we'd expect to give an overlapping instances error, but does not:

{-# LANGUAGE TypeFamilies, FlexibleInstances #-}
module Foo where
type family F a :: *
type instance F Bool = Int
type instance F Int = Bool
class C a where
instance (F Bool ~ a) => C a where
instance C Int where

Fortunately, GHC does notice when you actually try to use the instance.

Trac metadata

Trac field	Value
Version	7.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture