Confusing error messages with PolyKinds and superclasses

This program compiles fine:

{-# LANGUAGE FlexibleInstances #-}
import Data.Typeable

newtype Tagged t v = Tagged v
  deriving Typeable

class (Typeable t) => MyClass t where
  classF :: t -> Int

instance Typeable t => MyClass (Tagged t Int) where
  classF (Tagged n) = n

But if I add PolyKinds to the LANGUAGE pragma I get:

code/junk/typeable-problems.hs:17:10: error:
    • Could not deduce (Typeable k)
        arising from the superclasses of an instance declaration
      from the context: Typeable t
        bound by the instance declaration
        at code/junk/typeable-problems.hs:17:10-45
    • In the instance declaration for ‘MyClass (Tagged t Int)’
   |
17 | instance Typeable t => MyClass (Tagged t Int) where
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Which is very confusing since I don't have a k variable anywhere. Adding -fprint-explicit-kinds is somewhat better:

code/junk/typeable-problems.hs:17:10: error:
    • Could not deduce (Typeable * k)
        arising from the superclasses of an instance declaration
      from the context: Typeable k t
        bound by the instance declaration
        at code/junk/typeable-problems.hs:17:10-45
    • In the instance declaration for ‘MyClass (Tagged k t Int)’
   |
17 | instance Typeable t => MyClass (Tagged t Int) where
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

But still confusing. It doesn't really point me towards the solution - specifying that the kind of t is * explicitly - if I don't already know how the extension works.

It's also annoying that turning on an extension causes a type error, but I don't know if that's fixable.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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