#15125: Typeclass instance selection depends on the optimisation level -------------------------------------+------------------------------------- Reporter: nicuveo | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- (See the attached files for a minimal case.) A file A defines a typeclass, and gives an incoherent instance for all types a, and exports a function relying on said typeclass. A file B defines some data types, makes them specific instances of that class. Which instance ends up being picked depends on the optimisation level they're compiled with. //A.hs// {{{#!hs class A a where someValue :: a -> Maybe Int instance {-# INCOHERENT #-} A a where someValue = const Nothing getInt :: A a => a -> Int getInt x = fromMaybe 0 $ someValue x }}} //B.hs// {{{#!hs data B = B Int instance A B where someValue (B x) = Just x getBInt :: Int getBInt = getInt $ B 42 }}} //Main.hs// {{{#!hs main = print getBInt }}} **Upon compiling with -O0, this prints 42; upon compiling with -O2, this print 0.** (Interestingly, if the redundant class constraint is removed from getInt, then it always prints 0.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15125 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler