[bug] MesonToolchain does not respect libcxx setting

Environment Details (include every applicable attribute)

• Operating System+version: Ubuntu 22.04 LTS
• Compiler+version: Clang 13
• Conan version: 1.52.0
• Python version: 3.10.3

Steps to reproduce (Include if Applicable)

When running a verbose build, the -stdlib=libc++ option does not appear to be passed to Clang, causing the test package to fail with linking errors. As there are no other CCI packages using the new MesonToolchain to build C++ libraries, I’m not sure if this is a bug or if I am missing some generation step.

See libsigcpp recipe in https://github.com/conan-io/conan-center-index/pull/12775

Logs (Executed commands with output) (Include/Attach if Applicable)

[planetmarshall]

Workaround

(not comprehensive)

def generate(self):
      tc = MesonToolchain(self)
      if self.settings.compiler == "clang":
          libcxx = {
                  "libc++": ("-stdlib=libc++", None),
                  "libstdc++": ("-stdlib=libstdc++", 0),
                  "libstdc++11": ("-stdlib=libstdc++", 1)
                  }
          cxxflags, cxx11 = libcxx[str(self.settings.compiler.libcxx)]
          tc.cpp_args = [cxxflags]
          if cxx11 is not None:
              tc.cpp_args = [f"-D_GLIBCXX_USE_CXX11_ABI={cxx11}"]
              tc.c_args = [f"-D_GLIBCXX_USE_CXX11_ABI={cxx11}"]

      tc.generate()

[planetmarshall]

Test package created using

conan new <package> -t -m meson_lib

With a few modifications to run the test package using the new CMake toolchain.

conan-meson.zip

Steps to reproduce

1. Unzip the test package
2. Run conan create . -s compiler.libcxx=libc++ (with Clang)

Expected results

The test package is built correctly

Actual results

The test package fails with linking errors caused by linking a different C++ standard library to that used by the main package.