#12907: ExtendedDefaultRules-related regression in GHC 8.0.2 -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: high | Milestone: 8.0.2 Component: Compiler | Version: 8.0.2-rc1 (Type checker) | 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: -------------------------------------+------------------------------------- Commit https://git.haskell.org/ghc.git/commit/9a34bf1985035858ece043bf38b47b6ff4b88... introduced a regression between GHC 8.0.1 and 8.0.2 regarding the behavior of `ExtendedDefaultRules`. In GHC 8.0.1, this program {{{#!hs module Bug where default (Bool) }}} would compile without issue. On GHC 8.0.2, however, it is rejected with this error message: {{{ $ /opt/ghc/8.0.2/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:3:1: error: • The default type ‘Bool’ is not an instance of ‘Num’ • When checking the types in a default declaration }}} As a result, the `shelly` library now fails to compile with GHC 8.0.2 (see https://github.com/yesodweb/Shelly.hs/issues/130). A workaround is to explicitly enable `ExtendedDefaultRules` at the top of the module. Richard, do you understand what's going on here? I'm not sure if this is a real regression or just GHC being more particular (admittedly, I don't understand many of the intricacies of `ExtendedDefaultRules`). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12907 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler