"Iface type variable out of scope" with poly-kinded associated type and newtype-deriving

Summary

When a newtype-derived instance involving a kind-polymorphic associated type is called and optimized, ghc-9.4.2 prints the following error:

typecheckIface
Declaration for $fMyClassTYPEMyMaybe
Unfolding of $fMyClassTYPEMyMaybe:
  Iface type variable out of scope:  k

<no location info>: error:
    Cannot continue after interface file error

There are two parts to this error message: the Iface type variable out of scope part is a regression introduced in ghc-9.0, while the Cannot continue after interface file error part is a regression introduced in ghc-9.4.

I found several issues from years ago featuring this error message, triggered by increasingly-obscure circumstances. This is yet another obscure circumstance. #9263 (closed) is the most closely related, as it also involves associated types and polykinds, but not newtype-deriving. #2018 (closed) is also related in that it also requires optimizations to be turned on.

Note that some of those tickets say that the error only occurs the second time the code is built. This is not the case here, the error occurs the first time.

Steps to reproduce

The easiest way to reproduce the problem is to clone https://github.com/gelisam/ghc-9.4-bug and to run ./build.sh. Alternatively, create those two modules:

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
module MyModule where

import Data.Kind (Type)
import Data.Proxy (Proxy)

class MyClass (f :: k -> Type) where
  type MyTypeFamily f (i :: k) :: Type
  myMethod :: MyTypeFamily f i -> Proxy f -> Proxy i -> ()

instance MyClass Maybe where
  type MyTypeFamily Maybe i = ()
  myMethod = undefined

newtype MyMaybe a = MyMaybe (Maybe a)
  deriving MyClass

and

{-# LANGUAGE TypeApplications #-}
module Foo where

import Data.Kind (Type)
import Data.Proxy (Proxy(Proxy))

import MyModule

foo :: ()
foo = myMethod @Type @MyMaybe @() () Proxy Proxy

Then compile those two modules, with optimizations turned on:

$ ghc -O MyModule.hs Foo.hs
[2 of 2] Compiling Foo              ( Foo.hs, Foo.o )
typecheckIface
Declaration for $fMyClassTYPEMyMaybe
Unfolding of $fMyClassTYPEMyMaybe:
  Iface type variable out of scope:  k

<no location info>: error:
    Cannot continue after interface file error

Expected behavior

I expect the two modules to compile successfully.

Environment

• GHC version used: ghc-9.4.2, ghc-9.2.4, ghc-9.0.2

Of these, only ghc-9.4 fails to compile.

With ghc-9.2 and ghc-9.0, the Iface type variable out of scope message is printed but not Cannot continue after interface file error, and the module still compiles successfully.

I have also tried with ghc-8.10.7, which does not print either message.

Optional:

• Operating System: macOS Ventura 13.3
• System Architecture: arm64