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Opened by Ryan Scott@RyanGlScott
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Unused type variable error misidentifies the name of the type variable

If you compile the following (erroneous) program on GHC 8.8.2 or later:

{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
module Bug where

import Data.Kind

type Const (a :: Type) (b :: Type) = a

type family F (x :: a) :: Type where
  forall a x. F x = Const Int a

You'll get the following error:

$ /opt/ghc/8.8.2/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:11:10: error:
    • Type variable ‘a2’ is mentioned in the RHS,
        but not bound on the LHS of the family instance
    • In the equations for closed type family ‘F’
      In the type family declaration for ‘F’
   |
11 |   forall a x. F x = Const Int a
   |          ^

This points to the type variable a in the source code, but names it a2 in the error message! This appears to be the result of having a kind variable named a in the type family header, since renaming that kind variable to something else makes the issue go away. For example, this variant of the program above:

type family F (x :: b) :: Type where
  forall a x. F x = Const Int a

Gives a sensible error message:

$ /opt/ghc/8.8.2/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:11:10: error:
    • Type variable ‘a’ is mentioned in the RHS,
        but not bound on the LHS of the family instance
    • In the equations for closed type family ‘F’
      In the type family declaration for ‘F’
   |
11 |   forall a x. F x = Const Int a
   |          ^

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