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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.