[Bug c++/95968] New: error: 'args#0' is not a constant expression

["zakeria433 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95968

            Bug ID: 95968
           Summary: error: 'args#0' is not a constant expression
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zakeria433 at gmail dot com
  Target Milestone: ---

Hi,

command: g++ test_class.cpp -Wall -Wextra -std=c++20
System: Debian 10

The following code is rejected by Gcc 10.1. However, if I replace test_class's
constructor with constexpr instead of consteval, it does compile on gcc.

// Example program
#include <tuple>
#include <type_traits>
// #include <concepts>

template<typename T, typename ... Pack>
requires(std::is_floating_point<T>::value)
class test_class
{
public:
    const T first_param;
    const std::tuple <Pack...> pack;

    consteval test_class(T first_param_, Pack... pack_) :
first_param(first_param_), 
    pack(pack_...)
    {
    }

    consteval auto test_func()
    {
        //auto p = first_param;
        auto x = std::apply([/*p*/](const auto ... args) {return
test_class(1.0, args...);}, pack);
        return x;
    }
};

int main()
{
     constexpr auto var1 = test_class(4.0);
     //constexpr auto var2 = test_class(5.0);
     constexpr auto var3 = test_class(12.0, var1).test_func();
     //static_assert(vart3.first_param == 12, "assert fail");
}
error: ‘args#0’ is not a constant expression
   22 |         auto x = std::apply([/*p*/](const auto ... args) {return
test_class(1.0, args...);}, pack);
      |                                                                 
^~~~~~~~~~~~~~~~~~~~~~~~


Live demo:
https://godbolt.org/z/oWWFaP