Enforce compile-time constexpr [duplicate]

Question

In C++11 we get constexpr:

constexpr int foo (int x) {
    return x + 1;
}

Is it possible to make invocations of foo with a dynamic value of x a compile time error? That is, I want to create a foo such that one can only pass in constexpr arguments.

Answer 1

Yes, it can now be done in purely idiomatic C++, since C++20 added support for this kind of problem. You annotate a function with consteval and can be sure that it's being evaluated during compile time. https://en.cppreference.com/w/cpp/language/consteval

consteval int foo( int x ) {
    return x + 1;
}

int main( int argc, char *argv[] )
{
    return foo( argc ); // This will not compile
    return foo( 1 );    // This works
}

Also see this godbolt.org demo in the 3 most relevant compilers.