#11198: TypeInType error message regressions -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: high | Milestone: 8.0.1 Component: Compiler | Version: 7.11 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- `TypeInType` causes error messages around certain kind errors to degrade. Here is the case: {{{ [W] (alpha :: k) ~ (Int :: *) }}} where `alpha` is a unification variable, `k` is a skolem, and there is no witness that `k ~ *`. The solver will simplify to {{{ [W] kco :: k ~ * [W] (alpha :: k) ~ (Int |> sym kco) }}} and then the latter, now homogeneous equality is solved by unification {{{ alpha := Int |> sym kco }}} This is all sound. But when reporting a kind error around `k` not equalling `*`, GHC likes to report the type error that the kind error came from. But, after zonking, the type error looks like `Int ~ Int`, when we drop coercions (as we tend to do in error messages). That's unfortunate. It affects test cases * T9196 * T8262 * T8603 * tcfail122 Simon has proposed a rule that should avoid this: never, ever use a wanted as a kind cast. That might work, but an attempt at implementing caused a loop when kind families were involved. I will keep pushing on this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11198 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler