#11605: GHC accepts overlapping instances without pragma -------------------------------------+------------------------------------- Reporter: bennofs | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1-rc2 (Type checker) | Keywords: | Operating System: Linux Architecture: x86_64 | Type of failure: GHC accepts (amd64) | invalid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following code is accepted by GHC (I've tested versions 7.6.3, 7.8.4, 7.10.2, ghc 8.0.0.20160204 (rc2) and ghc 7.11.2015121 (git commit 28638dfe79e915f33d75a1b22c5adce9e2b62b97)), even though it obviously uses OverlappingInstances (yet the extension is not enabled, nor are any of the new overlappable/ing pragmas used). {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} import Data.Type.Equality import Data.Proxy import Debug.Trace class TypeEq a b where eqProofClass :: Maybe (a :~: b) instance TypeEq a b where eqProofClass = Nothing instance TypeEq a a where eqProofClass = Just Refl data Foo a = Foo a instance Eq a => Eq (Foo a) where Foo a1 == Foo a2 = case eqProofClass :: Maybe (a :~: Int) of Just Refl -> traceShow (a1 :: Int,a2) (a1 == a2) Nothing -> a1 == (undefined :: a) main :: IO () main = do print (Foo (1 :: Int) == Foo 2) putStrLn "--" print (Foo 'a' == Foo 'b') {- $output (1,2) False -- False -} }}} The example does no longer compiles if I add `TypeEq a Int` to the instance context, it then rightfully requests `OverlappingInstances` to be enabled. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11605 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler