{
  lowPrio,
  newScope,
  pkgs,
  lib,
  stdenv,
  cmake,
  preLibcCrossHeaders,
  substitute,
  substituteAll,
  fetchFromGitHub,
  fetchpatch,
  libxml2,
  python3,
  isl,
  fetchurl,
  overrideCC,
  wrapCCWith,
  wrapBintoolsWith,
  buildLlvmTools, # tools, but from the previous stage, for cross
  targetLlvmLibraries, # libraries, but from the next stage, for cross
  targetLlvm,
  # This is the default binutils, but with *this* version of LLD rather
  # than the default LLVM version's, if LLD is the choice. We use these for
  # the `useLLVM` bootstrapping below.
  bootBintoolsNoLibc ? if stdenv.targetPlatform.linker == "lld" then null else pkgs.bintoolsNoLibc,
  bootBintools ? if stdenv.targetPlatform.linker == "lld" then null else pkgs.bintools,
  darwin,
}:

let
  release_version = "12.0.1";
  candidate = ""; # empty or "rcN"
  dash-candidate = lib.optionalString (candidate != "") "-${candidate}";
  version = "${release_version}${dash-candidate}"; # differentiating these (variables) is important for RCs

  fetch =
    name: sha256:
    fetchurl {
      url = "https://github.com/llvm/llvm-project/releases/download/llvmorg-${version}/${name}-${release_version}${candidate}.src.tar.xz";
      inherit sha256;
    };

  clang-tools-extra_src = fetch "clang-tools-extra" "1r9a4fdz9ci58b5z2inwvm4z4cdp6scrivnaw05dggkxz7yrwrb5";

  inherit (import ../common/common-let.nix { inherit lib release_version; }) llvm_meta;

  tools = lib.makeExtensible (
    tools:
    let
      callPackage = newScope (
        tools
        // {
          inherit
            stdenv
            cmake
            libxml2
            python3
            isl
            release_version
            version
            fetch
            buildLlvmTools
            ;
        }
      );
      mkExtraBuildCommands0 = cc: ''
        rsrc="$out/resource-root"
        mkdir "$rsrc"
        ln -s "${cc.lib}/lib/clang/${release_version}/include" "$rsrc"
        echo "-resource-dir=$rsrc" >> $out/nix-support/cc-cflags
      '';
      mkExtraBuildCommands =
        cc:
        mkExtraBuildCommands0 cc
        + ''
          ln -s "${targetLlvmLibraries.compiler-rt.out}/lib" "$rsrc/lib"
          ln -s "${targetLlvmLibraries.compiler-rt.out}/share" "$rsrc/share"
        '';

      bintoolsNoLibc' = if bootBintoolsNoLibc == null then tools.bintoolsNoLibc else bootBintoolsNoLibc;
      bintools' = if bootBintools == null then tools.bintools else bootBintools;

    in
    {

      libllvm = callPackage ./llvm { inherit llvm_meta; };

      # `llvm` historically had the binaries.  When choosing an output explicitly,
      # we need to reintroduce `outputSpecified` to get the expected behavior e.g. of lib.get*
      llvm = tools.libllvm;

      libclang = callPackage ../common/clang {
        src = fetch "clang" "0px4gl27az6cdz6adds89qzdwb1cqpjsfvrldbz9qvpmphrj34bf";
        patches = [
          ./clang/purity.patch
          # https://reviews.llvm.org/D51899
          ./clang/gnu-install-dirs.patch
          (substituteAll {
            src = ../common/clang/clang-11-15-LLVMgold-path.patch;
            libllvmLibdir = "${tools.libllvm.lib}/lib";
          })
        ];
        inherit clang-tools-extra_src llvm_meta;
      };

      clang-unwrapped = tools.libclang;

      # disabled until recommonmark supports sphinx 3
      #Llvm-manpages = lowPrio (tools.libllvm.override {
      #  enableManpages = true;
      #  python3 = pkgs.python3;  # don't use python-boot
      #});

      clang-manpages = lowPrio (
        tools.libclang.override {
          enableManpages = true;
          python3 = pkgs.python3; # don't use python-boot
        }
      );

      # disabled until recommonmark supports sphinx 3
      # lldb-manpages = lowPrio (tools.lldb.override {
      #   enableManpages = true;
      #   python3 = pkgs.python3;  # don't use python-boot
      # });

      # pick clang appropriate for package set we are targeting
      clang =
        if stdenv.targetPlatform.libc == null then
          tools.clangNoLibc
        else if stdenv.targetPlatform.useLLVM or false then
          tools.clangUseLLVM
        else if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU then
          tools.libstdcxxClang
        else
          tools.libcxxClang;

      libstdcxxClang = wrapCCWith rec {
        cc = tools.clang-unwrapped;
        # libstdcxx is taken from gcc in an ad-hoc way in cc-wrapper.
        libcxx = null;
        extraPackages = [ targetLlvmLibraries.compiler-rt ];
        extraBuildCommands = mkExtraBuildCommands cc;
      };

      libcxxClang = wrapCCWith rec {
        cc = tools.clang-unwrapped;
        libcxx = targetLlvmLibraries.libcxx;
        extraPackages = [ targetLlvmLibraries.compiler-rt ];
        extraBuildCommands = mkExtraBuildCommands cc;
      };

      lld = callPackage ../common/lld {
        src = fetch "lld" "0qg3fgc7wj34hdkqn21y03zcmsdd01szhhm1hfki63iifrm3y2v9";
        patches = [ ./lld/gnu-install-dirs.patch ];
        inherit llvm_meta;
        inherit (libraries) libunwind;
      };

      lldb = callPackage ../common/lldb.nix {
        src = fetch "lldb" "0g3pj1m3chafavpr35r9fynm85y2hdyla6klj0h28khxs2613i78";
        patches =
          let
            resourceDirPatch = callPackage (
              { substituteAll, libclang }:
              substituteAll {
                src = ./lldb/resource-dir.patch;
                clangLibDir = "${libclang.lib}/lib";
              }
            ) { };
          in
          [
            ./lldb/procfs.patch
            resourceDirPatch
            ./lldb/gnu-install-dirs.patch
          ];
        inherit llvm_meta;
      };

      # Below, is the LLVM bootstrapping logic. It handles building a
      # fully LLVM toolchain from scratch. No GCC toolchain should be
      # pulled in. As a consequence, it is very quick to build different
      # targets provided by LLVM and we can also build for what GCC
      # doesn’t support like LLVM. Probably we should move to some other
      # file.

      bintools-unwrapped = callPackage ../common/bintools.nix { };

      bintoolsNoLibc = wrapBintoolsWith {
        bintools = tools.bintools-unwrapped;
        libc = preLibcCrossHeaders;
      };

      bintools = wrapBintoolsWith { bintools = tools.bintools-unwrapped; };

      clangUseLLVM = wrapCCWith rec {
        cc = tools.clang-unwrapped;
        libcxx = targetLlvmLibraries.libcxx;
        bintools = bintools';
        extraPackages = [
          targetLlvmLibraries.compiler-rt
        ] ++ lib.optionals (!stdenv.targetPlatform.isWasm) [ targetLlvmLibraries.libunwind ];
        extraBuildCommands =
          ''
            echo "-rtlib=compiler-rt -Wno-unused-command-line-argument" >> $out/nix-support/cc-cflags
            echo "-B${targetLlvmLibraries.compiler-rt}/lib" >> $out/nix-support/cc-cflags
          ''
          + lib.optionalString (!stdenv.targetPlatform.isWasm) ''
            echo "--unwindlib=libunwind" >> $out/nix-support/cc-cflags
            echo "-L${targetLlvmLibraries.libunwind}/lib" >> $out/nix-support/cc-ldflags
          ''
          + lib.optionalString (!stdenv.targetPlatform.isWasm && stdenv.targetPlatform.useLLVM or false) ''
            echo "-lunwind" >> $out/nix-support/cc-ldflags
          ''
          + lib.optionalString stdenv.targetPlatform.isWasm ''
            echo "-fno-exceptions" >> $out/nix-support/cc-cflags
          ''
          + mkExtraBuildCommands cc;
      };

      clangNoLibcxx = wrapCCWith rec {
        cc = tools.clang-unwrapped;
        libcxx = null;
        bintools = bintools';
        extraPackages = [ targetLlvmLibraries.compiler-rt ];
        extraBuildCommands =
          ''
            echo "-rtlib=compiler-rt" >> $out/nix-support/cc-cflags
            echo "-B${targetLlvmLibraries.compiler-rt}/lib" >> $out/nix-support/cc-cflags
            echo "-nostdlib++" >> $out/nix-support/cc-cflags
          ''
          + mkExtraBuildCommands cc;
      };

      clangNoLibc = wrapCCWith rec {
        cc = tools.clang-unwrapped;
        libcxx = null;
        bintools = bintoolsNoLibc';
        extraPackages = [ targetLlvmLibraries.compiler-rt ];
        extraBuildCommands =
          ''
            echo "-rtlib=compiler-rt" >> $out/nix-support/cc-cflags
            echo "-B${targetLlvmLibraries.compiler-rt}/lib" >> $out/nix-support/cc-cflags
          ''
          + mkExtraBuildCommands cc;
      };

      clangNoCompilerRt = wrapCCWith rec {
        cc = tools.clang-unwrapped;
        libcxx = null;
        bintools = bintoolsNoLibc';
        extraPackages = [ ];
        extraBuildCommands =
          ''
            echo "-nostartfiles" >> $out/nix-support/cc-cflags
          ''
          + mkExtraBuildCommands0 cc;
      };

      clangNoCompilerRtWithLibc = wrapCCWith rec {
        cc = tools.clang-unwrapped;
        libcxx = null;
        bintools = bintools';
        extraPackages = [ ];
        extraBuildCommands = mkExtraBuildCommands0 cc;
      };

    }
  );

  libraries = lib.makeExtensible (
    libraries:
    let
      callPackage = newScope (
        libraries
        // buildLlvmTools
        // {
          inherit
            stdenv
            cmake
            libxml2
            python3
            isl
            release_version
            version
            fetch
            ;
        }
      );
    in
    {

      compiler-rt-libc = callPackage ../common/compiler-rt {
        src = fetch "compiler-rt" "1950rg294izdwkaasi7yjrmadc9mzdd5paf0q63jjcq2m3rdbj5l";
        patches = [
          ../common/compiler-rt/7-12-codesign.patch # Revert compiler-rt commit that makes codesign mandatory
          ./compiler-rt/X86-support-extension.patch # Add support for i486 i586 i686 by reusing i386 config
          ./compiler-rt/gnu-install-dirs.patch
          # ld-wrapper dislikes `-rpath-link //nix/store`, so we normalize away the
          # extra `/`.
          ./compiler-rt/normalize-var.patch
          ../common/compiler-rt/darwin-plistbuddy-workaround.patch
          ./compiler-rt/armv7l.patch
          # Fix build on armv6l
          ../common/compiler-rt/armv6-mcr-dmb.patch
          ../common/compiler-rt/armv6-sync-ops-no-thumb.patch
          ../common/compiler-rt/armv6-no-ldrexd-strexd.patch
        ];
        inherit llvm_meta;
        stdenv =
          if stdenv.hostPlatform.useLLVM or false then
            overrideCC stdenv buildLlvmTools.clangNoCompilerRtWithLibc
          else
            stdenv;
      };

      compiler-rt-no-libc = callPackage ../common/compiler-rt {
        src = fetch "compiler-rt" "1950rg294izdwkaasi7yjrmadc9mzdd5paf0q63jjcq2m3rdbj5l";
        patches = [
          ../common/compiler-rt/7-12-codesign.patch # Revert compiler-rt commit that makes codesign mandatory
          ./compiler-rt/X86-support-extension.patch # Add support for i486 i586 i686 by reusing i386 config
          ./compiler-rt/gnu-install-dirs.patch
          # ld-wrapper dislikes `-rpath-link //nix/store`, so we normalize away the
          # extra `/`.
          ./compiler-rt/normalize-var.patch
          ../common/compiler-rt/darwin-plistbuddy-workaround.patch
          ./compiler-rt/armv7l.patch
          # Fix build on armv6l
          ../common/compiler-rt/armv6-mcr-dmb.patch
          ../common/compiler-rt/armv6-sync-ops-no-thumb.patch
          ../common/compiler-rt/armv6-no-ldrexd-strexd.patch
        ];
        inherit llvm_meta;
        stdenv =
          if stdenv.hostPlatform.useLLVM or false then
            overrideCC stdenv buildLlvmTools.clangNoCompilerRt
          else
            stdenv;
      };

      # N.B. condition is safe because without useLLVM both are the same.
      compiler-rt =
        if stdenv.hostPlatform.isAndroid then libraries.compiler-rt-libc else libraries.compiler-rt-no-libc;

      stdenv = overrideCC stdenv buildLlvmTools.clang;

      libcxxStdenv = overrideCC stdenv buildLlvmTools.libcxxClang;

      libcxx = callPackage ../common/libcxx {
        src = fetchFromGitHub {
          owner = "llvm";
          repo = "llvm-project";
          rev = "refs/tags/llvmorg-${version}";
          sparseCheckout = [
            "libcxx"
            "libcxxabi"
            "llvm/cmake"
            "llvm/utils"
            "runtimes"
          ];
          hash = "sha256-etxgXIdWxMTmbZ83Hsc0w6Jt5OSQSUEPVEWqLkHsNBY=";
        };
        patches =
          [
            (substitute {
              src = ../common/libcxxabi/wasm.patch;
              replacements = [
                "--replace-fail"
                "/cmake/"
                "/llvm/cmake/"
              ];
            })
          ]
          ++ lib.optionals stdenv.hostPlatform.isMusl [
            (substitute {
              src = ../common/libcxx/libcxx-0001-musl-hacks.patch;
              replacements = [
                "--replace-fail"
                "/include/"
                "/libcxx/include/"
              ];
            })
          ];
        inherit llvm_meta;
        stdenv = overrideCC stdenv buildLlvmTools.clangNoLibcxx;
      };

      libunwind = callPackage ../common/libunwind {
        src = fetch "libunwind" "192ww6n81lj2mb9pj4043z79jp3cf58a9c2qrxjwm5c3a64n1shb";
        patches = [ ./libunwind/gnu-install-dirs.patch ];
        inherit llvm_meta;
        stdenv = overrideCC stdenv buildLlvmTools.clangNoLibcxx;
      };

      openmp = callPackage ../common/openmp {
        src = fetch "openmp" "14dh0r6h2xh747ffgnsl4z08h0ri04azi9vf79cbz7ma1r27kzk0";
        patches = [
          # Fix cross.
          (fetchpatch {
            url = "https://github.com/llvm/llvm-project/commit/5e2358c781b85a18d1463fd924d2741d4ae5e42e.patch";
            hash = "sha256-UxIlAifXnexF/MaraPW0Ut6q+sf3e7y1fMdEv1q103A=";
          })
        ];
        inherit llvm_meta targetLlvm;
      };
    }
  );
  noExtend = extensible: lib.attrsets.removeAttrs extensible [ "extend" ];

in
{ inherit tools libraries release_version; } // (noExtend libraries) // (noExtend tools)