#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 RyanGlScott): The culprit appears to be [http://git.haskell.org/ghc.git/blob/ebb7613a66a784fc0015e13a6379453e6d5af44d... these lines] of `check_syn_tc_app`: {{{#!hs = do { -- See Note [Liberal type synonyms] ; liberal <- xoptM LangExt.LiberalTypeSynonyms ; if not liberal || isTypeFamilyTyCon tc then -- For H98 and synonym families, do check the type args mapM_ check_arg tys else -- In the liberal case (only for closed syns), expand then check case tcView ty of Just ty' -> let syn_tc = fst $ tcRepSplitTyConApp ty err_ctxt = text "In the expansion of type synonym" <+> quotes (ppr syn_tc) in addErrCtxt err_ctxt $ check_type env ctxt rank ty' Nothing -> pprPanic "check_tau_type" (ppr ty) } }}} When checking a type synonym application (like `Foo`) without `LiberalTypeSynonyms` enabled, then we don't check the expanded type at all—we only check the arguments! As a consequence, we never validity-check the underlying `Eq b => b` that is lurking behind the scenes in `f :: forall b (a :: Foo b). Int`. (This also reveals that enabling `LiberalTypeSynonyms` will cause the program to be rejected as well.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16059#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler