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#include <string>
#include <iostream>
#include <concepts>
//#include <type_traits>
template<typename Compound, typename Specified>
struct isEqual
{
enum { equal = false };
};
template<typename Specified>
struct isEqual<Specified, Specified>
{
enum { equal = true };
};
template<typename Compound, typename Specified>
concept same = isEqual<Compound, Specified>::equal;
//////////////////////////////////
template <typename Type, typename Ret>
concept ReturnType =
requires(Type par)
{
{ par[0] } -> std::same_as<Ret>; // par[..] returns a Ret
// no type constraints:
// { par } -> isEqual<Ret>::equal; // par returns a Ret
// { par << 1 } -> isEqual<decltype(par << 1), Ret>::equal;
// { par } -> same<Ret>;
// { par } noexcept;
{++par};
// same as:
// ++par;
};
///////////////////////////////
struct XX
{
void operator++()
{};
std::string operator[](int idx) const
{
return std::string{"a"};
}
};
template <typename Type, typename Ret>
requires ReturnType<Type, Ret>
Ret ret(Type tp)
{
return tp[0];
}
using namespace std;
int main()
{
ret<XX, string>(XX{});
}
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