Enforcing function contract at compile time when possible

Question

(this question was inspired by How can I generate a compilation error to prevent certain VALUE (not type) to go into the function?)

Let\'s say, we have a single-argument

Answer 1

gcc/clang/intel compilers support __builtin_constant_p, so you can use something like that:

template 
int foo_ub(int arg) {
    static_assert(D != 5, "error");
    int* parg = nullptr;
    if (arg != 5) {
        parg = &arg;
    }

    return *parg;
}

#define foo(e) foo_ub< __builtin_constant_p(e) ? e : 0 >(e)

these statements produce compile time error:

• foo(5)
• foo(2+3)
• constexpr int i = 5; foo(i);

while all others - runtime segfault (or ub if no nullptr is used)