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__has_extensionThe special operator __has_extension (operand) may be used in
constant integer contexts and in preprocessor ‘#if’ and ‘#elif’
expressions to test whether the identifier given in operand is recognized
as an extension supported by GCC given the current options. In any given
context, the features accepted by __has_extension are a strict superset
of those accepted by __has_feature. Unlike __has_feature,
__has_extension tests whether a given feature is available regardless of
strict language standards conformance.
If the -pedantic-errors flag is given, __has_extension is
equivalent to __has_feature.
Note that __has_feature and __has_extension are not recommended
for use in new code, and are only provided for compatibility with Clang. For
details of which identifiers are accepted by these function-like macros, see
the Clang documentation.