Variables pretty-printed from -ddump-deriv are not scoped properly

This bug is present on GHC 8.0.1, 8.0.2, 8.2.1 and HEAD, and originally noted in #13738 (closed)##13748. Take this code:

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE PolyKinds #-}
{-# OPTIONS_GHC -ddump-deriv #-}
module Works where

newtype Wrap f a = Wrap (f a) deriving C

class C f where
  c :: f a

When you compile it with GHC 8.0.2, you'll get this unsavory output:

GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Works            ( Works.hs, interpreted )

==================== Derived instances ====================
Derived instances:
  instance forall k_a14U (f_a14V :: k_a14U -> *).
           Works.C f_a14V =>
           Works.C (Works.Wrap f_a14V) where
    Works.c
      = GHC.Prim.coerce
          @(forall (a_a13F :: k_a14u). f_a13G a_a13F)
          @(forall (a_a13F :: k_a14u). Works.Wrap f_a13G a_a13F)
          Works.c
  

GHC.Generics representation types:

This is wrong, since the quantified variables in the instance head (k_a14U and f_a14V) do not match the occurrences that they bind (k_a14u and f_a13G). This is somewhat easier to see on GHC 8.2.1 or HEAD, since the binding sites are printed without uniques:

GHCi, version 8.3.20170516: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Works            ( Works.hs, interpreted )

==================== Derived instances ====================
Derived class instances:
  instance forall k (f :: k -> *).
           Works.C f =>
           Works.C (Works.Wrap f) where
    Works.c
      = GHC.Prim.coerce
          @(forall (a_a1tD :: k_a1uE). f_a1tE a_a1tD)
          @(forall (a_a1tD :: k_a1uE). Works.Wrap f_a1tE a_a1tD)
          Works.c
  

Derived type family instances:

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information

Variables pretty-printed from -ddump-deriv are not scoped properly

Child items ...

Activity