libstdcxxClang on macOS

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Is there a way to use libstdcxx with clang on macOS? The Nix code for libstdcxxClang seems to suggest that (since libcxx == null) that no attempt of using libstdcxx is done if the system is darwin.

I’m trying to do this to get -std=c++20 working, but Nixpkgs doesn’t seem to have a libcxx new enough for that.

What I’ve tried:

let gccShell = pkgs.mkShell.override {
		# Use gcc11 for our shell environment. We don't have a clang13Stdenv.
		stdenv = pkgs.gcc11Stdenv;
	};

	clang = pkgs.llvmPackages_latest.libstdcxxClang;
	clangFlags = ''
		-g
		-O1
		-DNDEBUG
		-pthread
		-std=c++20
		-I./
		-DUSING_TOMS_SUGGESTIONS
		-D__func__=__PRETTY_FUNCTION__
		-stdlib=libstdc++
		-Weverything
		-Wno-comma
		-Wno-unused-template
		-Wno-sign-conversion
		-Wno-exit-time-destructors
		-Wno-global-constructors
		-Wno-missing-prototypes
		-Wno-weak-vtables
		-Wno-padded
		-Wno-double-promotion
		-Wno-c++98-compat-pedantic
		-Wno-c++11-compat-pedantic
		-Wno-c++14-compat-pedantic
		-Wno-c++17-compat-pedantic
		-Wno-c++20-compat-pedantic
		-fdiagnostics-show-category=name
		-Wno-zero-as-null-pointer-constant
		-Wno-ctad-maybe-unsupported
	'';

in gccShell {
	buildInputs = with pkgs; [
		clang
		gcc11
	];
}

When built, I get errors related to headers not being found, like 'cmath' file not found, so clang is presumably not seeing the libstdcxx.