The C Preprocessor: __has_c_attribute

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#### 4.2.8 `__has_c_attribute`

The special operator `__has_c_attribute (operand)` may be used in ‘`#if`’ and ‘`#elif`’ expressions in C code to test whether the attribute referenced by its operand is recognized by GCC in attributes using the ‘`[[]]`’ syntax. GNU attributes must be specified with the scope ‘`gnu`’ or ‘`__gnu__`’ with `__has_c_attribute`. When operand designates a supported standard attribute it evaluates to an integer constant of the form `YYYYMM` indicating the year and month when the attribute was first introduced into the C standard, or when the syntax of operands to the attribute was extended in the C standard.