[C++] ValueOr(type) type check.

https://godbolt.org/z/qYG6rsY7h

#include <type_traits>
#include <utility>
template <typename T>
struct Foo {
    template <typename U>
    T ValueOr(U&&) {
        static_assert(sizeof(U) > 1);
        return {};
    }
};

template <typename T>
struct ForwardedDeclval {
    T operator()();
};
template <typename... Ts, typename F>
consteval void InstantiateForStaticAssertions(F f) {
    static_assert(std::is_same_v<decltype(f(ForwardedDeclval<Ts>()...)), void>);
}

template <typename T>
struct FooWrapper {
    template <typename U>
    T ValueOr(U&& u) {
        InstantiateForStaticAssertions<Foo<T>, decltype(u)>(
            [](auto t, auto u) { t().ValueOr(u()); });

        return {};
    }
};

int main() {
    FooWrapper<int> f;

    f.ValueOr(1);
    //f.ValueOr('5');
}