C++ concept simple requirements
/*
A simple requirement is an arbitrary expression statement that does not
start with the keyword requires. It asserts that the expression is valid.
The expression is an unevaluated operand; only language correctness is checked.
*/
template<typename T>
concept Addable = requires (T a, T b)
{
a + b; // "the expression a+b is a valid expression that will compile"
};
template<class T, class U = T>
concept Swappable = requires(T&& t, U&& u)
{
swap(std::forward<T>(t), std::forward<U>(u));
swap(std::forward<U>(u), std::forward<T>(t));
};
/*
A requirement that starts with the keyword requires is always interpreted
as a nested requirement. Thus a simple requirement cannot start with an
unparenthesized requires-expression.
*/