1 files changed, 53 insertions, 0 deletions
diff --git a/mesonbuild/coredata.py b/mesonbuild/coredata.py
index 97261d6..1184866 100644
--- a/mesonbuild/coredata.py
+++ b/mesonbuild/coredata.py
@@ -320,6 +320,59 @@ class UserFeatureOption(UserComboOption):
     def is_auto(self) -> bool:
         return self.value == 'auto'
 
+class UserStdOption(UserComboOption):
+    '''
+    UserOption specific to c_std and cpp_std options. User can set a list of
+    STDs in preference order and it selects the first one supported by current
+    compiler.
+
+    For historical reasons, some compilers (msvc) allowed setting a GNU std and
+    silently fell back to C std. This is now deprecated. Projects that support
+    both GNU and MSVC compilers should set e.g. c_std=gnu11,c11.
+
+    This is not using self.deprecated mechanism we already have for project
+    options because we want to print a warning if ALL values are deprecated, not
+    if SOME values are deprecated.
+    '''
+    def __init__(self, lang: str, all_stds: T.List[str]) -> None:
+        self.lang = lang.lower()
+        self.all_stds = ['none'] + all_stds
+        # Map a deprecated std to its replacement. e.g. gnu11 -> c11.
+        self.deprecated_stds: T.Dict[str, str] = {}
+        super().__init__(f'{lang} language standard to use', ['none'], 'none')
+
+    def set_versions(self, versions: T.List[str], gnu: bool = False, gnu_deprecated: bool = False) -> None:
+        assert all(std in self.all_stds for std in versions)
+        self.choices += versions
+        if gnu:
+            gnu_stds_map = {f'gnu{std[1:]}': std for std in versions}
+            if gnu_deprecated:
+                self.deprecated_stds.update(gnu_stds_map)
+            else:
+                self.choices += gnu_stds_map.keys()
+
+    def validate_value(self, value: T.Union[str, T.List[str]]) -> str:
+        candidates = UserArrayOption.listify_value(value)
+        unknown = [std for std in candidates if std not in self.all_stds]
+        if unknown:
+            raise MesonException(f'Unknown {self.lang.upper()} std {unknown}. Possible values are {self.all_stds}.')
+        # Check first if any of the candidates are not deprecated
+        for std in candidates:
+            if std in self.choices:
+                return std
+        # Fallback to a deprecated std if any
+        for std in candidates:
+            newstd = self.deprecated_stds.get(std)
+            if newstd is not None:
+                mlog.deprecation(
+                    f'None of the values {candidates} are supported by the {self.lang} compiler.\n' +
+                    f'However, the deprecated {std} std currently falls back to {newstd}.\n' +
+                    'This will be an error in the future.\n' +
+                    'If the project supports both GNU and MSVC compilers, a value such as\n' +
+                    '"c_std=gnu11,c11" specifies that GNU is prefered but it can safely fallback to plain c11.')
+                return newstd
+        raise MesonException(f'None of values {candidates} are supported by the {self.lang.upper()} compiler. ' +
+                             f'Possible values are {self.choices}')
 
 class DependencyCacheType(enum.Enum):