How can I define a concept that is satisfied by an arbitrary std::vector?

Question

I would like to have a concept requiring an arbitrary vector as the return type:

template<typename T>
concept HasVector = requires (T t) {
    { T.vec() } -> std::same_as<std::vector<int>>; //works
    { T.vec() } -> std::same_as<std::vector<foo>>; //want to put something arbitrary in here
}

Such that we would have something like the following:

class A {
std::vector<int> vec() { /* ... */}
}

class B {
std::vector<double> vec() { /* ... */}
}

static_assert(HasVector<A>);
static_assert(HasVector<B>);

Moreover, it would be even nicer to require a vector as the return type whose value type satisfies some other concept, i.e.

template<typename T>
concept Arithmetic = // as in the standard

template<typename T>
concept HasArithmeticVector = requires (T t ) {
    { T. vec() } -> std::same_as<std::vector<Arithmetic>>;

Is there such a way to put this in names of concepts?

Answer 1

We start by writing a variable template to check if a type specializes a template:

template <typename T, template <typename...> class Z>
inline constexpr bool is_specialization_of = false;

template <template <typename...> class Z, class... Args>
inline constexpr bool is_specialization_of<Z<Args...>, Z> = true;

Which we can turn into a concept:

template <typename T, template <typename...> class Z>
concept Specializes = is_specialization_of<T, Z>;

Which we can then use to implement another concept:

template<typename T>
concept HasVector = requires (T t) {
    { t.vec() } -> Specializes<std::vector>;
};

If you want to then do further checking, that's just adding more requirements.

template<typename T>
concept HasVector = requires (T t) {
    { t.vec() } -> Specializes<std::vector>;

    // or something along these lines
    requires Arithmetic<decay_t<decltype(t.vec()[0])>>;
    requires Arithmetic<range_value_t<decltype(t.vec())>>;
    // etc.
};