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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

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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Architecture