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

It's not perfect and it requires us to use arguments in two different places, but it 'works':

template
int foo(int arg = 0) {
    static_assert(N != 5, "N cannot be 5!");
    int* parg;
    if (arg != 5) {
        parg = &arg;
    }

    return *parg;
}

We can call it like so:

foo<5>();   // does not compile
foo(5);     // UB
foo<5>(5);  // does not compile
foo<5>(10); // does not compile
foo<10>(5); // UB
foo();      // fine
foo<10>();  // fine
foo(10);    // fine