c++: add support for c++23/c++26 standard and aliases

GCC 12.3 and Clang 16 support -std flags for c++23/c++2b. The unreleased
GCC 14 and Clang 17 will support -std flags for c++26/c++2c.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>

mesonbuild/compilers/cpp.py

@@ -226,6 +226,10 @@ class _StdCPPLibMixin(CompilerMixinBase):
 
 
 class ClangCPPCompiler(_StdCPPLibMixin, ClangCompiler, CPPCompiler):
+
+    _CPP23_VERSION = '>=12.0.0'
+    _CPP26_VERSION = '>=17.0.0'
+
     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,
@@ -252,11 +256,18 @@ class ClangCPPCompiler(_StdCPPLibMixin, ClangCompiler, CPPCompiler):
             ),
             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')
+        if version_compare(self.version, self._CPP26_VERSION):
+            cppstd_choices.append('c++26')
+            cppstd_choices.append('gnu++26')
+        opts[key.evolve('std')].choices = cppstd_choices
         if self.info.is_windows() or self.info.is_cygwin():
             opts.update({
                 key.evolve('winlibs'): coredata.UserArrayOption(
@@ -298,7 +309,11 @@ class ArmLtdClangCPPCompiler(ClangCPPCompiler):
 
 
 class AppleClangCPPCompiler(ClangCPPCompiler):
-    pass
+
+    _CPP23_VERSION = '>=13.0.0'
+    # TODO: We don't know which XCode version will include LLVM 17 yet, so
+    # use something absurd.
+    _CPP26_VERSION = '>=99.0.0'
 
 
 class EmscriptenCPPCompiler(EmscriptenMixin, ClangCPPCompiler):
@@ -418,6 +433,9 @@ class GnuCPPCompiler(_StdCPPLibMixin, GnuCompiler, CPPCompiler):
         if version_compare(self.version, '>=12.2.0'):
             cppstd_choices.append('c++23')
             cppstd_choices.append('gnu++23')
+        if version_compare(self.version, '>=14.0.0'):
+            cppstd_choices.append('c++26')
+            cppstd_choices.append('gnu++26')
         opts[key].choices = cppstd_choices
         if self.info.is_windows() or self.info.is_cygwin():
             opts.update({