GHC bug with TemplateHaskell + DuplicateRecordFields

1_readme.md

This is a minimal reproducing example for bug in GHC. It is observed in 8.6.3 (Stack LTS-13.6), 8.6.5 (nix) and 8.8.1 (nix).

$ ghc Exp.hs -ddump-splices
[1 of 2] Compiling TH               ( TH.hs, TH.o )
[2 of 2] Compiling Exp              ( Exp.hs, Exp.o )
Exp.hs:11:1-10: Splicing declarations make ''Bar ======>

Exp.hs:11:1: error:
    Ambiguous occurrence ‘Exp.foo’
    It could refer to
       either the field ‘foo’, defined at Exp.hs:9:18
           or the field ‘foo’, defined at Exp.hs:8:18
   |
11 | make ''Bar
   | ^^^^^^^^^^

TH function make only calls reify and does nothing, as is seen from -ddump-splices. However, reify on record fields triggers a very weird error with DuplicateRecordFields.

Exp.hs

{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE TemplateHaskell #-}

module Exp where

import           TH

-- Define two records with the same field
data Foo = Foo { foo :: String, bla :: Int}
data Bar = Bar { foo :: String, bar :: Bool }

-- Call TH function that calls reify and returns nothing
make ''Bar

TH.hs

module TH where

import           Language.Haskell.TH

make :: Name -> Q [Dec]
make name = do
  -- reify type name to get record constructor and fields
  TyConI (DataD _ _ _ _ [RecC con fields'] _) <- reify name
  let fields = [ field | (field, _, _) <- fields' ]

  -- now suppose we need to reify fields to get their types...
  fieldInfos <- mapM reify fields

  -- and do nothing more
  return []