Default methods don't pass implicit kind parameters properly

When compiling the following module:

{-# 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 #13998 (closed) .

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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