Can Consteval Functions Legalize Function Parameters in Template Parameters Dependent on Function Arguments?

Consteval Functions: Limitations on Template Parameters Dependent on Function Arguments

In C 17, defining constexpr functions that rely on template parameters dependent on function arguments was prohibited. While consteval functions in C 20 promise to enforce compile-time evaluation, does this mean that such code can now be legalized?

Question:

consteval int foo(int i) { return std::integral_constant::value; }. Will this code be valid in C 20?

Answer:

No.

Explanation:

Despite the introduction of consteval functions, the underlying principle remains that non-template function definitions have a single point of type resolution. Legalizing the proposed code would have implications for the One Definition Rule (ODR). Additionally, the C 20 paper explicitly states that function parameters will not be treated as core constant expressions due to potential type mismatches.

As a result, function parameters cannot be directly used as constant expressions within template parameter dependencies, even in consteval function contexts.

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