#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): Phab:D2682 Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Simon, that sounds sensible to me. I was quite shocked myself to learn how many `DefaultSignatures` validity checks are shunted off until the site of instance declarations instead of the class declaration, and moreover, that there is no requirement that the normal type signature and the default type signature must be the same (modulo context differences). Of course, that all sounds wonderful in my head, but if tracking down these kinds of regressions have taught me anything, it's that fiddling with `DefaultSignatures` further is inevitably bound to break more code in the wild. I don't say that to discourage you from pursuing this change (which I think is a net positive), but be aware that there are probably Haskell programs that are abusing `DefaultSignatures` in wildly creative ways, so we'll likely step on their toes in some way. '''tl;dr''' If we change the typechecking rules for `DefaultSignatures` further, we should carefully consider whether it's worth introducing in a minor release :) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12784#comment:16 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler