#14883: QuantifiedConstraints don't kick in when used in TypeApplications -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.5 checker) | Keywords: Resolution: | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: 15290 | Blocking: Related Tickets: #15290 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * related: => #15290 Comment: In light of https://ghc.haskell.org/trac/ghc/ticket/15290#comment:5, perhaps it's best that we not pursue making the impredicative variants typecheck. If we incorporate the change to `deriving` that Simon suggests in that ticket, then the code that would be generated for the original program would be: {{{#!hs instance Traversable' m => Traversable' (T1 m) where traverse' :: forall f a b. (Applicative' f) => (a -> f b) -> T1 m a -> f (T1 m b) traverse' = coerce @((a -> f b) -> m a -> f (m b)) traverse' }}} Which, conveniently enough, is pretty much exactly the `-- Typechecks` variant! So that's convenient. I'll keep this ticket open since as a reminder to check in these two lovely test cases, but implementing the `deriving` change in #15290 should make this whole issue moot. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14883#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler