C++ 20 Concepts/Requires clauses

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I would be really grateful if somebody could explain how C++ 20+ compilers (MSVC 2022 in my case) are able to compile the following, why does the Simple concept have no effect?

template <typename T>
concept Simple = requires(T t)
{
    std::is_trivial_v<T> == true;
};


void foo(Simple auto s) {
    std::cout << "bar";
}


int main(int argc, char** argv)
{
    using Bytes = std::span<std::byte>;

    Bytes b;

    static_assert(false == std::is_trivial_v<Bytes>);

    foo(b);  //compiles and prints "bar"
}
c++20 c++
1 Answers
template <typename T>
concept Simple = requires(T t)
{
    std::is_trivial_v<T> == true;
};

This checks if expression std::is_trivial_v<T> == true is well-formed, ignoring its value.

To check if the expression is truthy, add a nested requires:

template <typename T>
concept Simple = requires(T t)
{
    requires std::is_trivial_v<T>/* == true*/;
};

Or just put it outside of requires:

template <typename T>
concept Simple = std::is_trivial_v<T>;
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