__is_constexpr()

/*
 * This returns a constant expression while determining if an argument is
 * a constant expression, most importantly without evaluating the argument.
 * Glory to Martin Uecker <Martin.Uecker@med.uni-goettingen.de>
 */
#define __is_constexpr(x) \
        (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))

See also

• https://stackoverflow.com/questions/49481217/linux-kernels-is-constexpr-macro
• https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3c8ba0d61d04ced9f8d9ff93977995a9e4e96e91
• https://lkml.org/lkml/2018/3/20/845