#12201: Wrong instance selection with overlapping instances and local bindings -------------------------------------+------------------------------------- Reporter: kanetw | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by kanetw: @@ -1,1 +1,1 @@ - test and test2 should have the same type, but it seems like the {-# + test and test2 should have the same type, but it seems like the non-{#- New description: test and test2 should have the same type, but it seems like the non-{#- OVERLAPPING #-} instance is chosen prematurely. {{{#!hs {-# LANGUAGE NoMonomorphismRestriction, FlexibleContexts, FlexibleInstances #-} module Bug where class A a where class A a => B a where data Foo s a instance {-# OVERLAPPING #-} A (Foo Int Bool) instance A (Foo s a) instance B (Foo s a) bar :: (A a, B a) => a bar = undefined helper :: Foo s Bool -> s -> Int helper = undefined {- *Bug> :t test test :: A (Foo s Bool) => s -> Int -} test = helper bar {- *Bug> :t test2 test2 :: s -> Int -} test2 = let foo = bar in helper foo }}} Compare with 7.10.3: {{{ *Bug> :t test test :: A (Foo s Bool) => s -> Int *Bug> :t test2 test2 :: A (Foo s Bool) => s -> Int }}} -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12201#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler