I’m observing some weird behavior, which is probably closely related to issue #15149 (closed).

The following minimized code does not compile on 8.2.1, 8.2.2, but it does compile on 8.4.3.

However, in my original (non-free) codebase, this is reversed: 8.2.2 compiles it just fine, and 8.4.3 fails with both errors per usage location at the same time:

src/.../Docs.hs:123:11: error:
    • Constructor ‘X'Y’ does not have the required strict field(s): z

...

src/.../Docs.hs:123:11: error:
    • Constructor ‘X'Y’ does not have field ‘z’

I noticed, after updating the codebase’s compiler to 8.4.3.

If the z field is renamed and unique, it compiles correctly.

How can I go about debugging/minimizing this?

This is the minimized code that works the other way round (OK on 8.4.3, fails on 8.2.2):

{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE StrictData #-}

module Main where

data AB = A | B

class SomeClass (ab :: AB) where
  data SomeData ab

instance SomeClass 'A where
  data SomeData 'A = SomeData'A{someField :: Int} deriving Show

instance SomeClass 'B where
  data SomeData 'B = SomeData'B{someField :: Int}

main :: IO ()
main = print SomeData'A{someField = 5}