check class template instantiations belong to same class template

Question

How to check if two class template instantiations belong to same class template. This is my code

#include <iostream>
#include <type_traits>

template<typename T1, typename T2>
class A {
    float val;
public:

};

int main() {
    A<double, double> a_float_type;
    A<int, int> a_int_type;

    // how to check whether both a_double_type and a_int_type were instantiated from the same template class A<,>
    std::cout << std::is_same<decltype(a_float_type), decltype(a_int_type)>::value << std::endl; // returns false
}

My compiler only supports C++11

Answer 1

This is surprisingly quite simple:

template<typename T1, typename T2>
struct form_same_template : std::false_type
{};

template<template<typename...> class C, typename ... TAs, typename ...TBs>
struct form_same_template<C<TAs...>, C<TBs...>> : std::true_type
{};

#if __cplusplus > 201103L
template<typename T1, typename T2>
constexpr bool form_same_template_v = form_same_template<T1, T2>::value;
#endif

https://godbolt.org/z/hq65PKqKr

As you can see it passes basic tests.

Some tweaks are needed if you wish to handle templates with none type template parameters (it fails on std::array - I've added test for it).