#12784: Typechecker regression in GHC 8.0.2 involving DefaultSignatures -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: highest | Milestone: 8.0.2 Component: Compiler | Version: 8.1 Resolution: | 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: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * cc: bgamari (added) Comment: rwbarton, if I understand correctly, the issue here is that we're trying to equate `t m` with `m`? (It sort of looks like a variation of the occurs check, but I don't know if it's the same thing, given the nature of the error message.) It certainly seems like this program shouldn't have typechecked before, and that's fine. But the other issue is that we have a program which compiles on 8.0.1 but fails with 8.0.2, which seems a bit iffy. (See also https://ghc.haskell.org/trac/ghc/ticket/12768#comment:5 for another example.) Should we avoid backporting this fix to 8.0.2 to give users time to migrate their code that exhibits this bug before the next major release (8.2) lands? Or should we just pull the trigger and add a blurb to the 8.0.2 release notes explaining the situation? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12784#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler