#14861: QuantifiedConstraints: Can't use forall'd variable in context -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.5 checker) | Keywords: Resolution: | QuantifiedConstraints wipT2893 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: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Replying to [comment:1 simonpj]:
Can you say why?
When we pattern-match on `ECC1`, we bring into scope the given constraint `c p0`, where `p0` is a fresh type variable. We have as a constraint: {{{#!hs (forall p. c p => Foldable' f) }}} GHC should be able to use the given `c p0` constraint to discharge the obligation and conclude `Foldable' f`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14861#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler