#16141: StrictData and TypeFamilies regression -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.8.1 Component: Compiler | Version: 8.6.3 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The credit goes to wuzzeb for originally discovering this bug [https://www.reddit.com/r/haskell/comments/ad9a7k/strictdata_typefamilies_and... here]. I've minimized their test case slightly below: {{{#!hs {-# LANGUAGE StrictData #-} {-# LANGUAGE TypeFamilies #-} module Bug where data family T newtype instance T = MkT Int deriving Eq }}} With optimization enabled, this program compiles with GHC 8.0.2 through 8.4.4, but not with 8.6.3 or HEAD: {{{ $ /opt/ghc/8.4.4/bin/ghc -fforce-recomp -O Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) $ /opt/ghc/8.6.3/bin/ghc -fforce-recomp -O Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:6:39: error: • Couldn't match a lifted type with an unlifted type arising from the coercion of the method ‘==’ from type ‘GHC.Prim.Int# -> GHC.Prim.Int# -> Bool’ to type ‘T -> T -> Bool’ • When deriving the instance for (Eq T) | 6 | newtype instance T = MkT Int deriving Eq | ^^ }}} Based on the error message, it appears as if GHC mistakenly believes that the representation type of the `T` instance is `Int#`, rather than `Int`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16141 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler