T.126 Use a common function for runtime/compile-time evaluation, and `std::is_constant_evaluated()` if necessary

Here is a suggested guideline for std::is_constant_evaluated(), which is currently not mentioned anywhere.

The main motivation is to avoid code duplication, and to unify the interface of constexpr/non-constexpr functions.

This suggestion could also be phrased in terms of if consteval, but C++23 is still not widespread.

T.126 Use a common function for runtime/compile-time evaluation, and std::is_constant_evaluated() if necessary

Reason

Avoid code duplication, avoid refactoring when implementing more functionality in constexpr functions.

Example, good

constexpr double pow(double base, int exp)
{
    if (std::is_constant_evaluated()) {
        double result = 1;
        for (int i = 0; i < exp; ++i) {
            result *= base;
        }
        return result;
    }
    else return std::pow(base, exp);
}

// good, runtime and compile-time evaluation use the same function
constexpr double pi_cubed = pow(std::numbers::pi, 3);
double pi_cubed = pow(std::numbers::pi, 3);

Example, bad

constexpr double constexpr_pow(double base, int exp)
{
    double result = 1;
    for (int i = 0; i < exp; ++i) {
        result *= base;
    }
     return result;
}

// bad, runtime and compile-time use different functions that do the same
// if std::pow is made constexpr, we may have to replace all uses of constexpr_pow
constexpr double e_squared = constexpr_pow(std::numbers::e, 2);
double e_squared = std::pow(std::numbers::e, 2);

Note

In C++23, write if consteval instead of if (std::is_constant_evaluated()).

Warning

Do not write if constexpr (std::is_constant_evaluated()). This is a common mistake and the condition is always true.

Enforcement

Flag separate runtime/compile-time versions of the same function. Merge runtime function into compile-time function.

No file or test is named in the issue. Start by locating the existing guideline entries and reviewing nearby rule wording; done means incorporating T.126 with its rationale, examples, note, warning, and enforcement guidance.