#16394: GHC internal error while typechecking of instance definition -------------------------------------+------------------------------------- Reporter: Day1721 | Owner: (none) Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.6.3 (Type checker) | Keywords: | Operating System: Linux Architecture: | Type of failure: Incorrect Unknown/Multiple | error/warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Hello. \\ This code won't typecheck because of GHC internal error. {{{#!hs {-# LANGUAGE PolyKinds, TypeFamilies, DataKinds #-} class C a where type T (n :: a) -- v--DIFF--v instance C a => C b => C (a, b) where type T '(n, m) = (T n, T m) }}} with error message: {{{ Bug.hs:7:10: error: • GHC internal error: ‘T’ is not in scope during type checking, but it passed the renamer tcl_env of environment: [a1LS :-> Type variable ‘a’ = a :: *, a1LT :-> Type variable ‘b’ = b :: *] • In the type instance declaration for ‘T’ In the instance declaration for ‘C b => C (a, b)’ | 7 | type T (n, m) = (T n, T m) | ^ Failed, no modules loaded. }}} but this works fine: {{{#!hs {-# LANGUAGE PolyKinds, TypeFamilies, DataKinds #-} class C a where type T (n :: a) -- v--DIFF--v instance (C a, C b) => C (a, b) where type T '(n, m) = (T n, T m) }}} Not sure is a bug, but either way it would be better to make more understandable error message -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16394 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler