objc: Fix handling of c_std option with fallback

This is only a stopgap, as a clean solution would require some
refactoring.

mesonbuild/compilers/objc.py

@@ -9,6 +9,7 @@
 from ..mesonlib import OptionKey
 
 from .compilers import Compiler
+from .c import _ALL_STDS, _ClangCStds
 from .mixins.clike import CLikeCompiler
 from .mixins.gnu import GnuCompiler, gnu_common_warning_args, gnu_objc_warning_args
 from .mixins.clang import ClangCompiler
@@ -74,6 +75,7 @@ def __init__(self, ccache: T.List[str], exelist: T.List[str], version: str, for_
         ObjCCompiler.__init__(self, ccache, exelist, version, for_machine, is_cross,
                               info, exe_wrapper, linker=linker, full_version=full_version)
         ClangCompiler.__init__(self, defines)
+        self._clang_objcstds = _ClangObjCStds(version, for_machine)
         default_warn_args = ['-Wall', '-Winvalid-pch']
         self.warn_args = {'0': [],
                           '1': default_warn_args,
@@ -83,13 +85,7 @@ def __init__(self, ccache: T.List[str], exelist: T.List[str], version: str, for_
 
     def get_options(self) -> 'coredata.MutableKeyedOptionDictType':
         opts = super().get_options()
-        opts.update({
-            OptionKey('std', machine=self.for_machine, lang='c'): coredata.UserComboOption(
-                'C language standard to use',
-                ['none', 'c89', 'c99', 'c11', 'c17', 'gnu89', 'gnu99', 'gnu11', 'gnu17'],
-                'none',
-            )
-        })
+        opts.update(self._clang_objcstds.get_options())
         return opts
 
     def get_option_compile_args(self, options: 'coredata.KeyedOptionDictType') -> T.List[str]:
@@ -99,6 +95,30 @@ def get_option_compile_args(self, options: 'coredata.KeyedOptionDictType') -> T.
             args.append('-std=' + std.value)
         return args
 
+class _ClangObjCStdsBase:
+    language = 'c'
+
+    def __init__(self, version: str, for_machine: MachineChoice):
+        self.version = version
+        self.for_machine = for_machine
+
+    def get_options(self) -> 'coredata.MutableKeyedOptionDictType':
+        key = OptionKey('std', machine=self.for_machine, lang=self.language)
+        return {key: coredata.UserStdOption('C', _ALL_STDS)}
+
+class _ClangObjCStds(_ClangCStds, _ClangObjCStdsBase):
+    def __init__(self, version: str, for_machine: MachineChoice):
+        _ClangObjCStdsBase.__init__(self, version, for_machine)
+
+    def get_output_args(self, outputname: str) -> T.List[str]:
+        pass
+
+    def get_optimization_args(self, optimization_level: str) -> T.List[str]:
+        return []
+
+    def sanity_check(self, work_dir: str, environment: 'Environment') -> None:
+        pass
+
 class AppleClangObjCCompiler(ClangObjCCompiler):
 
     """Handle the differences between Apple's clang and vanilla clang."""