#9867: PatternSynonyms + ScopedTypeVariables triggers an internal error -------------------------------------+------------------------------------- Reporter: antalsz | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- When trying to give a type signature with a free type variable to a pattern in a pattern synonym, GHC dies with an internal error. Given the file {{{#!hs {-# LANGUAGE PatternSynonyms, ScopedTypeVariables #-} pattern Nil = [] :: [a] }}} the resulting error is {{{ Example.hs:2:22: GHC internal error: ‘a’ is not in scope during type checking, but it passed the renamer … tcl_env of environment: [] In an expression type signature: [a] In the expression: [] :: [a] In an equation for ‘$WNil’: $WNil = [] :: [a] Compilation failed. }}} The fix is to add `RankNTypes` and an explicit `forall`: {{{#!hs {-# LANGUAGE PatternSynonyms, ScopedTypeVariables, RankNTypes #-} pattern Nil = [] :: forall a. [a] }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9867 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler