c++: add support for c++23 standard and aliases

GCC 12.3 and Clang 16 support -std flags for c++23/c++2b.

Signed-off-by: Steven Noonan <[email protected]>
Signed-off-by: Patrick Williams <[email protected]>
Backported-by: Nirbheek Chauhan <[email protected]>

Author: tycho

mesonbuild/compilers/cpp.py

@@ -224,6 +224,9 @@ def language_stdlib_only_link_flags(self, env: Environment) -> T.List[str]:
 
 
 class ClangCPPCompiler(_StdCPPLibMixin, ClangCompiler, CPPCompiler):
+
+    _CPP23_VERSION = '>=12.0.0'
+
     def __init__(self, ccache: T.List[str], exelist: T.List[str], version: str, for_machine: MachineChoice, is_cross: bool,
                  info: 'MachineInfo', exe_wrapper: T.Optional['ExternalProgram'] = None,
                  linker: T.Optional['DynamicLinker'] = None,
@@ -250,11 +253,15 @@ def get_options(self) -> 'MutableKeyedOptionDictType':
             ),
             key.evolve('rtti'): coredata.UserBooleanOption('Enable RTTI', True),
         })
-        opts[key.evolve('std')].choices = [
+        cppstd_choices = [
             'none', 'c++98', 'c++03', 'c++11', 'c++14', 'c++17', 'c++1z',
             'c++2a', 'c++20', 'gnu++11', 'gnu++14', 'gnu++17', 'gnu++1z',
             'gnu++2a', 'gnu++20',
         ]
+        if version_compare(self.version, self._CPP23_VERSION):
+            cppstd_choices.append('c++23')
+            cppstd_choices.append('gnu++23')
+        opts[key.evolve('std')].choices = cppstd_choices
         if self.info.is_windows() or self.info.is_cygwin():
             opts.update({
                 key.evolve('winlibs'): coredata.UserArrayOption(
@@ -296,7 +303,8 @@ class ArmLtdClangCPPCompiler(ClangCPPCompiler):
 
 
 class AppleClangCPPCompiler(ClangCPPCompiler):
-    pass
+
+    _CPP23_VERSION = '>=13.0.0'
 
 
 class EmscriptenCPPCompiler(EmscriptenMixin, ClangCPPCompiler):