Reproducer for clang_linux-64 and clangxx_linux-64 build failure

conda_build_config.yaml

c_compiler:
  - clang
cxx_compiler:
  - clangxx
c_stdlib:
  - sysroot                    # [linux]
  - macosx_deployment_target   # [osx]
  - vs                         # [win]

main.cpp

#include <iostream>
#include <optional>
#include <string>

int main() {
    std::optional<std::string> msg = std::string("optional-ok");
    if (!msg) {
        return 1;
    }
    std::cout << *msg << '\n';
    return 0;
}

pixi.toml

[workspace]
name = "cpp17-optional-reproducer"
description = "Minimal C++17 std::optional compiler/stdlib sanity check"
channels = ["conda-forge"]
platforms = ["linux-64"]

[dependencies]
rattler-build = "*"

[tasks]
# Reproduces the breakage: libstdcxx-devel_linux-64 is NOT added to build requirements
build-broken = "rattler-build build --recipe . --variant 'pin_libstdcxx_devel=false' --output-dir output"

# Fixes the issue: libstdcxx-devel_linux-64 15.* IS added to build requirements
build-fixed = "rattler-build build --recipe . --variant 'pin_libstdcxx_devel=true' --output-dir output"

recipe.yaml

context:
  name: cpp17-optional-reproducer
  version: "0.1.0"

package:
  name: ${{ name }}
  version: ${{ version }}

source:
  path: .
  use_gitignore: false

build:
  number: 0
  skip:
    - win
  script:
    # Helpful diagnostics in the build log
    - echo "CC=${CC}"
    - echo "CXX=${CXX}"
    - ${CXX} -std=c++17 -v -E -x c++ /dev/null
    # Actual compile (this is where <optional> will fail if stdlib headers are broken)
    - ${CXX} -std=c++17 -O2 main.cpp -o optional-smoke
    - mkdir -p ${PREFIX}/bin
    - cp optional-smoke ${PREFIX}/bin/optional-smoke

requirements:
  build:
    - ${{ compiler('c') }}
    - ${{ stdlib('c') }}
    - ${{ compiler('cxx') }}
    # Controlled via -m variant-fixed.yaml / variant-broken.yaml (use_libstdcxx_devel)
    - if: pin_libstdcxx_devel == "true"
      then:
        - libstdcxx-devel_linux-64 15.*

tests:
  - script:
      - optional-smoke
      - test "$(optional-smoke)" = "optional-ok"

about:
  summary: "Minimal C++17 std::optional compiler/stdlib sanity check"
  license: BSD-3-Clause