#14288: ScopedTypeVariables with nested foralls broken since 8.0.2 -------------------------------------+------------------------------------- Reporter: MikolajKonarski | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.1 checker) | Resolution: | Keywords: Operating System: Linux | Architecture: x86_64 Type of failure: GHC rejects | (amd64) valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Actually, the issue I mentioned about pattern synonyms applies //today//. That it, this typechecks: {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE PatternSynonyms #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeApplications #-} data T a where MkT :: forall a b. (Show b) => a -> b -> T a pattern ExNumPat :: forall a. (Num a, Eq a) => forall b. (Show b) => b -> T a pattern ExNumPat x <- MkT 42 x where ExNumPat x = MkT @a 42 x }}} But this (with `@b`) doesn't! {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE PatternSynonyms #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeApplications #-} data T a where MkT :: forall a b. (Show b) => a -> b -> T a pattern ExNumPat :: forall a. (Num a, Eq a) => forall b. (Show b) => b -> T a pattern ExNumPat x <- MkT 42 x where ExNumPat x = MkT @a @b 42 x }}} Moreover, there's no workaround for this, since one can't combine the two `forall`s in the pattern signature for the reasons discussed in comment:6. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14288#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler