Can concepts be used to put a constraint on values as well as types?

Question

Concepts can be used to put a constraint on types as template parameters like the example below:

template<typename t, int v>
concept the_concept1 = sizeof(t) > v;

template<int v, the_concept1<v> t>
struct some_struct1{};

I am trying to use a similar method with values like the example below:

template<int v1, int v2>
concept the_concept2 = v1 > v2;

template<int v1, the_concept2<v1> v2>
struct some_struct2{};

But with G++ 10 I am getting the following error message:

error: ‘the_concept2’ does not constrain a type

So I was wondering if concepts can be used to put a constraint on values? If so then how should I do it?

Edit: My final goal is to use the concept in declaration of a template structure with variadic template parameters like:

template<typename t, std::size_t ... v>
struct the_struct;

And I need a concept to check if every v is less than sizeof(t).

Answer 1

If you want to use a concept as a named type constraint on a template parameter, as in your example, the concept needs to apply to a type template parameter.

You can still define concepts that apply only to e.g. non-type template parameters, however, as long as you use it in a context which allows these; e.g. using a requires-clause:

template<int v1, int v2>
concept the_concept2 = v1 > v2;

template<int v1, int v2> requires the_concept2<v1, v2>
struct some_struct2{};

using valid = some_struct2<42, 41>;
//using invalid = some_struct2<42, 42>; // constraints not satisfied

Another example applied on a function template or a member function of a class template:

template<int v1, int v2>
concept the_concept2 = v1 > v2;

template <int a, int b>
void bar() requires the_concept2<a, b> {} 

template <int a, int b>
struct Foo {
    static void bar() requires the_concept2<a, b> {} 
};

int main() {
    bar<2, 1>();
    Foo<2, 1>::bar();
    //bar<2, 2>();      // candidate template ignored: constraints not satisfied
    //Foo<2, 2>::bar(); // invalid reference to function 'bar': constraints not satisfied
}

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The following OP edit (which basically asks an entirely different question)

Edit: My final goal is to use the concept in declaration of a template structure with variadic template parameters like:

template<typename t, std::size_t ... v>
struct the_struct;

And I need a concept to check if every v is less than sizeof(t).

can be achieved by specifying the concept itself to apply for variadic non-type template parameters that are expanded in the sizeof(T) > v check using parameter pack expansion:

#include <cstddef>
#include <cstdint>

template<typename T, std::size_t... v>
concept the_concept1 = (... && (sizeof(T) > v));

template<typename T, std::size_t... vs> requires the_concept1<T, vs...>
struct the_struct;

using TypeOfSize4Bytes = uint32_t;

using valid = the_struct<TypeOfSize4Bytes, 1, 3, 2, 1>;
using also_valid = the_struct<TypeOfSize4Bytes>;
//using invalid = the_struct<TypeOfSize4Bytes, 1, 2, 4>;  // error: constraints not satisfied