When you want to build with clang, generally it seems good to use overrideCC stdenv clang instead of the gcc stdenv. At least for my use cases it removed the need for hacks like these

# This compiler is not using the Nix wrappers, so it needs some help to find things.

I see, but I think the situation is different here. Swift is largely built by gcc I believe. But while building Swift, its build system also compiles modified versions of LLVM and clang, which it later also runs during the build. For this local clang, these hacks are needed.

I don't know, but I had checked that it is using clang from nixpkgs in some way, too. EDIT: in particular

    # However, we want to use the wrapped compiler whenever possible.
    export CC="${clang}/bin/clang"

Yes. I must admit I do not really understand where and how that passed-in clang is used. I have been trying to update to Swift 5.2.4. I have to change things there anyways, so I can try your suggestion as well.

@vcunat yeah by all accounts the thing is a mess, I just was trying to find the quickest way to keep the mess working, and have things be cleaned up later.

Should we just merge this, then?