[tom_]

The enum idea is interesting. I've previously used an extern with a conditional size of either 1 (valid) or -1 (invalid). This requires no additional boilerplate, and is #define-able into a static assert when built with a recent enough compiler. Something like this, from memory:

    #define STATIC_ASSERT(COND) extern char static_assert_cond_[(COND)?1:-1] /* C99 or earlier */
    #define STATIC_ASSERT(COND) _Static_assert(COND) /* C11 or later */

As both are declarations, I don't think you'll end up in a situation where one is valid and the other isn't - but I could be wrong, and I suspect it would rarely matter in practice anyway.