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#include <concepts>
#include <type_traits>
// REQUIRES EXPRESSION
// requires { requirement-seq }
// requires ( parameter-list ) { requirement-seq }
//
// [1] https://en.cppreference.com/w/cpp/language/requires
// [2] https://en.cppreference.com/w/cpp/language/constraints#Constraints
//
// REQUIREMENT CLAUSE
// Not the same as a REQUIREMENT EXPRESSIONS, and is used to require
// constraints (express concept bounds).
//
// [1] https://en.cppreference.com/w/cpp/language/constraints#Requires_clauses
// -- HELPER -------------------------------------------------------------------
template<typename T>
using Alias = T;
void print(int);
// -- CONCEPTS & REQUIRE EXPRESSIONS -------------------------------------------
// Simple concept from a type trait.
template<typename T, typename U>
concept Same = std::is_same<T, U>::value;
// Simple requirement concepts.
template<typename T>
concept TraitAddAndPrint = requires(T t, int i) {
// Adding T + int must be supported.
t + i;
// Calling print(T) must be available.
print(t);
};
// Type requirement concepts.
template<typename T>
concept TraitTypes = requires(T t) {
// T must have a type definition inner.
typename T::inner;
// Type alias must exist.
typename Alias<T>;
};
// Compound requirement concepts.
template<typename T>
concept TraitFns = requires(T t, const T c) {
// void T::foo() must exist.
{ t.foo() };
// bool T::bar() const; must exist.
{ c.bar() } -> Same<bool>;
// static void T::stat(); must exist.
{ T::stat() } -> Same<int>;
};
// Nested requirement concepts.
template<typename T>
concept TraitNested = requires(T t) {
// Must satisfy other concepts.
requires TraitTypes<T>;
requires TraitFns<T>;
};
// -- REQUIRE EXPRESSIONS ------------------------------------------------------
// Require expressions can be evaluated to booleans.
template<typename T>
static constexpr bool IsTraitFns = requires { requires TraitFns<T>; };
// Require expressions can also be used in static assertions.
static_assert(requires { requires Same<int, int>; });
static_assert(!requires {
typename Alias<int>;
requires Same<int, void>;
});
// -- TESTS --------------------------------------------------------------------
static_assert(requires { requires TraitAddAndPrint<int>; });
struct FnTypeGood {
using inner = int;
};
struct FnTypeBad {};
static_assert(requires { requires TraitTypes<FnTypeGood>; });
static_assert(!requires { requires TraitTypes<FnTypeBad>; });
struct FnGood {
void foo();
bool bar() const;
static int stat();
};
struct FnBad {};
static_assert(requires { requires TraitFns<FnGood>; });
static_assert(!requires { requires TraitFns<FnBad>; });
struct NestedGood : FnTypeGood, FnGood {};
struct NestedBad1 : FnGood {};
struct NestedBad2 : FnTypeGood {};
static_assert(requires { requires TraitNested<NestedGood>; });
static_assert(!requires { requires TraitNested<NestedBad1>; });
static_assert(!requires { requires TraitNested<NestedBad2>; });
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