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  • Glasgow Haskell CompilerGlasgow Haskell Compiler
  • GHCGHC
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  • #9288
Closed
Open
Issue created Jul 09, 2014 by Edward Z. Yang@ezyangDeveloper

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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