Can Function Parameters Be Template Dependent Because of Consteval Functions?

Can Consteval Functions Enable Template Parameters Dependent on Function Arguments?

In C 17, defining a constexpr function that returns a compile-time constant may seem logical, but it's prohibited. The compiler demands runtime execution instructions, preventing template instantiation involving such functions.

C 20 introduces consteval functions, ensuring their evaluation at compile time. With this, many expected this constraint to vanish, allowing code like:

<code class="cpp">consteval int foo(int i) {
    return std::integral_constant<int, i>::value;
}</code>

However, the answer is still a resolute no.

Despite the paper's potential changes, the fact remains that non-template function definitions are typed only once. Permitting such code would potentially enable the declaration of variables with non-ODR-friendly types like std::integral_constant, which is highly undesirable.

The paper also implies that parameters shouldn't be treated as core constant expressions. One example illustrates this:

<code class="cpp">consteval int sqrsqr(int n) {
  return sqr(sqr(n)); // Not a constant-expression at this  point,
}                     // but that's okay.</code>

In summary, function parameters will not evolve into constant expressions due to potential typing inconsistencies.

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