Function template with concept is instanciated even if the constraint is not met #130242
Description
The following code is OK in MSVC, but not in clang.
#include <iostream>
#include <optional>
template <typename T>
concept HasVoid = std::is_void_v<typename T::Type>;
template <typename T>
concept HasNonVoid = !std::is_void_v<typename T::Type>;
struct Void
{
using Type = void;
};
struct NonVoid
{
using Type = int;
};
template <typename T, std::enable_if_t<HasVoid<T>, int> = 0>
void UseEnableIf(bool value)
{
std::cout << "Void" << std::endl;
}
template <typename T, std::enable_if_t<HasNonVoid<T>, int> = 0>
void UseEnableIf(const std::optional<typename T::Type>& value)
{
std::cout << "NonVoid" << std::endl;
}
template <HasVoid T>
void UseConcept(bool value)
{
std::cout << "Void" << std::endl;
}
template <HasNonVoid T>
void UseConcept(const std::optional<typename T::Type>& value)
{
std::cout << "NonVoid" << std::endl;
}
int main()
{
UseEnableIf<Void>(true); // OK
UseEnableIf<NonVoid>(1);
UseConcept<Void>(true); // clang: error, MSVC: OK
UseConcept<NonVoid>(1);
}
In clang, UseConcept<Void>(true); causes errors like std::optional<void> is illegal.
It seems both the first and the second UseConcept functions are instanciated, even if the second functions's constraint is not met.
On the other hand, it works in MSVC.
I don't think the concept is a perfect replacement for std::enable_if_t,
but which behavior is correct according to the C++ standard?
clang version: 18.1.8 (Red Hat, Inc. 18.1.8-3.el9)
C++ standard: C++20