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 | |
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Blocking | |
CC | |
Operating system | |
Architecture |