Hello. \\ This code won't typecheck because of GHC internal error.

{-# LANGUAGE PolyKinds, TypeFamilies, DataKinds #-}

class C a where
    type T (n :: a)

--       v--DIFF--v
instance C a => C b => C (a, b) where
    type T '(n, m) = (T n, T m)

with error message:

Bug.hs:7:10: error:
    • GHC internal error: ‘T’ is not in scope during type checking, but it passed the renamer
      tcl_env of environment: [a1LS :-> Type variable ‘a’ = a :: *,
                               a1LT :-> Type variable ‘b’ = b :: *]
    • In the type instance declaration for ‘T’
      In the instance declaration for ‘C b => C (a, b)’
  |
7 |     type T (n, m) = (T n, T m)
  |          ^
Failed, no modules loaded.

but this works fine:

{-# LANGUAGE PolyKinds, TypeFamilies, DataKinds #-}

class C a where
    type T (n :: a)

--       v--DIFF--v
instance (C a, C b) => C (a, b) where
    type T '(n, m) = (T n, T m)

Not sure is a bug, but either way it would be better to make more understandable error message