#16059: checkValidType is defeated by a type synonym -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler (Type | Version: 8.7 checker) | Resolution: | 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: | -------------------------------------+------------------------------------- Comment (by goldfire): The problem in this code {{{#!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 }}} is the ''definition'' of `Foo`, not the usage of it. As comment:3 illustrates and comment:4 elaborates, we don't really want to check the expansion of a type synonym in this way. (Hold off on considering `LiberalTypeSynonyms` for now.) Even with the rather transparent interpretation of type synonyms in GHC, I think they should offer ''some'' level of abstraction (in contrast to the position in comment:4): it seems that allowing type synonyms at least to hide what extensions are enabled is sensible. What about `LiberalTypeSynonyms`? I don't know. But let's see if we can agree on conservative synonyms first, and then we'll move to liberal ones. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16059#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler