#16059: checkValidType is defeated by a type synonym -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler | Version: 8.7 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC accepts Unknown/Multiple | invalid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- GHC throws an error message on this code: {{{#!hs {-# LANGUAGE DataKinds #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE RankNTypes #-} module Bug where f :: forall b (a :: Eq b => b). Int f = 42 }}} {{{ $ ~/Software/ghc/inplace/bin/ghc-stage2 --interactive Bug.hs GHCi, version 8.7.20181211: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:6:6: error: • Illegal constraint in a kind: Eq b => b • In the type signature: f :: forall b (a :: Eq b => b). Int | 6 | f :: forall b (a :: Eq b => b). Int | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} However, it does //not// throw an error message if you hide `Eq b => b` behind a type synonym, as the following variant of the code above is accepted: {{{#!hs {-# LANGUAGE DataKinds #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE RankNTypes #-} module Bug where type Foo b = Eq b => b f :: forall b (a :: Foo b). Int f = 42 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16059 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler