Can Consteval Functions Enable Template Parameter Dependence on Function Arguments in C 20?

Consteval Functions and Template Parameter Dependence on Function Arguments

In C 17, using a function that returns a constant directly within a template parameter declaration is prohibited due to runtime execution constraints. With the introduction of consteval functions in C 20, these constraints are expected to be removed. However, will this enable the following code to compile successfully?

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

No.

Despite the consteval function's compile-time evaluation requirement, the function definition itself only undergoes typing once. As such, function parameters will never be treated as core constant expressions due to the potential for typing discrepancies.

This limitation is further emphasized in the C 20 proposal, which explicitly excludes parameters from being considered constant expressions in its 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>

Therefore, consteval functions do not allow template parameters to depend on function arguments because parameters lack the necessary typing guarantees as constant expressions.

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