#7854: Constrained method type accepted in Haskell 98 mode -------------------------------------+------------------------------------- Reporter: refold | Owner: thomie Type: bug | Status: patch Priority: normal | Milestone: 7.12.1 Component: Compiler (Type | Version: 7.6.3 checker) | Keywords: newcomer Resolution: | Architecture: Operating System: Unknown/Multiple | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | typechecker/should_fail/T7854 Related Tickets: #10118, #10119 | Blocking: | Differential Revisions: Phab:D688 -------------------------------------+------------------------------------- Comment (by simonpj): Thomie, thanks for working on this. Inspired by your patches I can see a simpler way to achieve the same goal, so I'll do that. Specifically I plan to do this: * Fix the original bug simply by always doing a `checkValidType` on the entire selector-id type. That includes an ambiguity check which is precisely what we need. * Implement the check when `ConstrainedClassMethods` is ''off'' as a special check that does nothing else. * As comment:4 acknowledges, GHC has not been making the claimed check for ages. Rather than making `ConstrainedClassMethods` on by default (as you propose), I plan to make it implied by `MultiParamTypeClasses`, which means that we are out of Haskell-98 land anyway. This means that some programs may break, but (a) the error message suggests adding `ConstrainedClassMethods` (b) the fix is easy. If necessary (e.g. on a Stackage run) we can make it on-by-default in 7.10 to ease transition. Commit coming. Thanks for unblocking this. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7854#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler