#13998: Default Signature messes up arity of type constructor -------------------------------------+------------------------------------- Reporter: andrewthad | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * cc: RyanGlScott (added) Comment: It works on GHC 8.0.1 and earlier, but not on 8.0.2 or later. This can be explained by the fact that the way GHC typechecks default implementations received a drastic overhaul in 8.0.2. Unfortunately, that's been the root of many bugs... That being said, I have a hunch what is going on here. From inspecting the `-ddump-tc-trace` output, one can observe that GHC gives the following type signature for `$dmeqForallPoly`: {{{ tcInferId Main.$dmeqForallPoly :: forall k (f :: k -> *). EqForallPoly f => forall (a :: k) (b :: k). TestEquality f => f a -> f b -> Bool }}} Notice that the `k` comes //before// `f`. However, in the generated `EqForallPoly Value` instance (which can be seen with `-ddump-deriv`): {{{ Main.EqForallPoly [Main.Atom, Main.Value] Main.eqForallPoly = Main.$dmeqForallPoly @(Main.Value) }}} It's using `TypeApplications` to instantiate `k` to `Value`. This is wrong, however, since we really want to be instantiating //`f`// to `Value`. That is, we'd rather generate this code: {{{ instance EqForallPoly Value where eqForallPoly = $dmeqForallPoly @Atom @Value }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13998#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler