Behaviour of DerivingVia is different between GHC 8.6.3 and GHC 8.6.4

May relate: #15376

{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE DerivingVia #-}
{-# LANGUAGE StandaloneDeriving #-}

module TestSpace.PolyKinds where

import Control.Monad.Trans.Reader

newtype T m a = T (Int -> m a)

deriving via (ReaderT Int m) instance Functor m => Functor (T m)

with transformers is failed to compile on GHC 8.6.3 and is succeeded on 8.6.4 .

However, this is very sensitive. I got this example on cabal project, the behaviour is affected by whether to run cabal new-freeze and version of transformers.

Sample is here: https://github.com/mizunashi-mana/maybreak-derivingvia

My environment:

• OS: macOS Mojave 10.14.3
• CPU: Intel Core i7