================================================================================
Concept definition
================================================================================

template <class T, class U>
concept Derived = std::is_base_of<U, T>::value;
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================================================================================
Concept definition with requires expression
================================================================================

template<typename T>
concept Hashable = requires(T a) {
    { std::hash<T>{}(a) } -> std::convertible_to<std::size_t>;
};
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================================================================================
Requires clauses and expressions
================================================================================

template<typename T>
void f(T&&) requires Eq<T>; // can appear as the last element of a function declarator

template<typename T> requires Addable<T> // or right after a template parameter list
T add(T a, T b) { return a + b; }

template<typename T>
concept Addable = requires (T x) { x + x; }; // requires-expression

template<typename T>
    requires requires (T x) { x + x; } // ad-hoc constraint, note keyword used twice
T add(T a, T b) { return a + b; }

template<typename T>
    requires (!std::is_same_v<T, bool>) // parenthesized expressions are allowed
void f(T);

template<typename T> requires Addable<T> && Subtractable<T> // conjunctions
T f(T);

template<typename T> requires Addable<T> and Subtractable<T> // conjunctions
T f(T);

template<typename T> requires Addable<T> || Subtractable<T> // disjunctions
T f(T);

template<typename T> requires Addable<T> or Subtractable<T> // conjunctions
T f(T);

template<typename T> requires false || true // boolean literals
T f(T);

template<typename... T> requires (... && Addable<T>) // fold expressions
T f(T);

--------------------------------------------------------------------------------

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================================================================================
Compound requirements
================================================================================

template<typename T> concept C2 =
requires(T x) {
    {*x} -> std::convertible_to<typename T::inner>; // the expression *x must be valid
                                                    // AND the type T::inner must be valid
                                                    // AND the result of *x must be convertible to T::inner
    {x + 1} -> std::same_as<int>; // the expression x + 1 must be valid
                               // AND std::same_as<decltype((x + 1)), int> must be satisfied
                               // i.e., (x + 1) must be a prvalue of type int
    {x * 1} -> std::convertible_to<T>; // the expression x * 1 must be valid
                                       // AND its result must be convertible to T
};

--------------------------------------------------------------------------------

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================================================================================
Nested requirements
================================================================================

template <class T>
concept Semiregular = DefaultConstructible<T> &&
    CopyConstructible<T> && Destructible<T> && CopyAssignable<T> &&
requires(T a, size_t n) {
    requires Same<T*, decltype(&a)>;  // nested: "Same<...> evaluates to true"
    { a.~T() } noexcept;  // compound: "a.~T()" is a valid expression that doesn't throw
    requires Same<T*, decltype(new T)>; // nested: "Same<...> evaluates to true"
    requires Same<T*, decltype(new T[n])>; // nested
    { delete new T };  // compound
    { delete new T[n] }; // compound
};
--------------------------------------------------------------------------------

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================================================================================
Constraints
================================================================================

template <EqualityComparable T>
void f(const T&); // constrained function template declaration

void f(const EqualityComparable auto&); // constrained function template declaration

Sortable auto foo = f();
Sortable<T> auto bar = g();
NS::Concept<T> auto baz = h();

Sortable decltype(auto) foo = i();

---

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