Can Consteval Functions Allow Template Parameters Reliant on Function Arguments?

Can Consteval Functions Enable Template Parameters Reliant on Function Arguments?

In C 17, constexpr functions like the below snippet are invalid:

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

Despite foo's evaluation at compile-time, the compiler requires it to be executable at runtime, hindering template instantiation.

C 20 introduces consteval functions, which enforce compile-time evaluation. One may wonder if this allows code like the following:

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

The answer is no.

The paper's potential changes cannot alter the singular typing of non-template function definitions. Furthermore, if this code were valid, it would open up the possibility of declaring variables of type std::integral_constant, which seems highly restrictive in terms of the One Definition Rule (ODR).

The paper also illustrates that parameters will not be treated as core constant expressions through an example:

<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 essence, function parameters will always lack constant expression status due to potential typing inconsistencies.

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