`-Wtype-equality-requires-operators` warning building Cabal-syntax with ghc-9.4

[jneira]

After adding lot of allow-newer building Cabal-syntax with ghc-9.4 throws this warning:

Preprocessing library for Cabal-syntax-3.9.0.0..
Building library for Cabal-syntax-3.9.0.0..
....
src\Distribution\Compat\Graph.hs:180:37: error: [-Wtype-equality-requires-operators, -Werror=type-equality-requires-operators]
    The use of ‘~’ without TypeOperators
    will become an error in a future GHC release.
    Suggested fix: Perhaps you intended to use TypeOperators
    |
180 | instance (IsNode a, IsNode b, Key a ~ Key b) => IsNode (Either a b) where
    |

[andreasabel]

TypeOperators should be added to the default-extensions of the .cabal files (Cabal-syntax.cabal, ..). This is harmless because the language flag exists since ages (before GHC 7).

[jneira]

hmm just i've checked the module has the language declared explicitly:

cabal/Cabal-syntax/src/Distribution/Compat/Graph.hs

Line 7 in 5159ffb

{-# LANGUAGE TypeOperators #-}

so a ghc bug?

[jneira]

so a ghc bug?

No, the error is building Cabal-3.6.3.0 and it did not have the language:

cabal/Cabal/src/Distribution/Compat/Graph.hs

Lines 1 to 7 in 6046687

	{-# LANGUAGE BangPatterns #-}
	{-# LANGUAGE CPP #-}
	{-# LANGUAGE DeriveDataTypeable #-}
	{-# LANGUAGE FlexibleContexts #-}
	{-# LANGUAGE ScopedTypeVariables #-}
	{-# LANGUAGE TypeFamilies #-}
	{-# LANGUAGE UndecidableInstances #-}

[jneira]

well not sure if it worths to backport the code which makes a warning go away so closing