libstdcxxClang on macOS

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 = ''

in gccShell {
	buildInputs = with pkgs; [

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