> But what business does the bootstrap compiler have with compiling GHC.Internal.Classes?

Aha! I see the issue.

If you look here: https://gitlab.haskell.org/ghc/ghc/-/blob/master/libraries/ghc-boot-th/ghc-boot-th.cabal.in#L55

    if flag(bootstrap)
        cpp-options: -DBOOTSTRAP_TH
        build-depends:
            ghc-prim
        hs-source-dirs: @SourceRoot@ ../ghc-internal/src
        exposed-modules:
            GHC.Boot.TH.Lib
            GHC.Boot.TH.Syntax
            GHC.Boot.TH.Monad
        other-modules:
            GHC.Internal.TH.Lib
            GHC.Internal.TH.Syntax
            GHC.Internal.TH.Monad
            GHC.Internal.ForeignSrcLang
            GHC.Internal.LanguageExtensions
            GHC.Internal.Lexeme

You can see the modules that we vendor from `ghc-internal` under `other-modules.

One of those is `GHC.Internal.ForeignSrcLang` and in your PR you add:

```

import GHC.Internal.Classes( (==) ) -- For known-key names

```

This needs to go inside the `else` branch of the `ifdef BOOTSTRAP_TH` pragma.

I think if you do the same thing for any `ghc-internal` imports in the other modules mentioned above then you should be fine.

I really should add a note explaining this BOOTSTRAP_TH business, especially because it is quite hacky(!).

Note [Bootstrapping Template Haskell] does a good job of explaining the design decisions we made to get here, but we need another note to explain this CPP soup.

Cheers,

Teo