Re: [GHC] #14434: GHC 8.2.1 picks wrong instances with -O in a presence of an OVERLAPPABLE instance

#14434: GHC 8.2.1 picks wrong instances with -O in a presence of an OVERLAPPABLE instance -------------------------------------+------------------------------------- Reporter: dredozubov | Owner: (none) Type: bug | Status: new Priority: high | Milestone: 8.2.3 Component: Compiler | Version: 8.2.1 Resolution: | Keywords: Operating System: Linux | Architecture: x86_64 Type of failure: Incorrect result | (amd64) at runtime | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones ): In [changeset:"30058b0e45e920319916be999de9c4d77da136e7/ghc" 30058b0e/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="30058b0e45e920319916be999de9c4d77da136e7" Fix another dark corner in the shortcut solver The shortcut solver for type classes (Trac #12791) was eagerly solving a constaint from an OVERLAPPABLE instance. It happened to be the only one in scope, so it was unique, but since it's specfically flagged as overlappable it's really a bad idea to solve using it, rather than using the Given dictionary. This led to Trac #14434, a nasty and hard to identify bug. }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14434#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler