#15120: Default methods don't pass implicit kind parameters properly -------------------------------------+------------------------------------- Reporter: mbieleck | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Keywords: PolyKinds, | Operating System: Unknown/Multiple DefaultSignatures | Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When compiling the following module: {{{#!hs {-# LANGUAGE PolyKinds #-} {-# LANGUAGE DefaultSignatures #-} module TestCase where import Data.Proxy class Describe a where describe :: Proxy a -> String default describe :: Proxy a -> String describe _ = "" data Foo = Foo instance Describe Foo }}} I get the following error (on GHC 8.0.2 and 8.2.2, with `-fprint-explicit- kinds`): {{{ TestCase.hs:15:10: error: • Couldn't match type ‘*’ with ‘Foo’ Expected type: Proxy * Foo -> String Actual type: Proxy Foo Foo -> String • In the expression: TestCase.$dmdescribe @Foo In an equation for ‘describe’: describe = TestCase.$dmdescribe @Foo In the instance declaration for ‘Describe * Foo’ | 15 | instance Describe Foo | ^^^^^^^^^^^^ }}} The Core generated for `$dmdescribe` has the following type signature: {{{ TestCase.$dmdescribe :: forall k (a :: k). Describe k a => Proxy k a -> String }}} I believe the failure results from the fact that the type application `TestCase.$dmdescribe @Foo` passes `Foo` as the `k` parameter instead of `a`. Seems related to https://ghc.haskell.org/trac/ghc/ticket/13998 . -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15120 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler