#10619: Order matters when type-checking -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by goldfire): It gets worse. The same problem happens between these two: This succeeds: {{{ {-# LANGUAGE RankNTypes #-} module Bug where foo _ = if True then ((\x -> x) :: (forall a. a -> a) -> forall b. b -> b) else \y -> y }}} This fails: {{{ foo _ = if True then \y -> y else ((\x -> x) :: (forall a. a -> a) -> forall b. b -> b) }}} But the cause is different. This second case is caused by the fact that `ReturnTv`s can unify with polytypes, and the `ReturnTv` for the whole expression is just pushed into the `if`'s branches. But even if the `ReturnTv` were replaced by a `TauTv` before pushing into the `if`'s branches, it would ''still'' be broken because a `TauTv` with an !OpenKind can also unify with polytypes. I think the solution for this case is to create a new flavor of `TauTv` that truly insists on being a tau-type. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10619#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler