Error compilation of casting an enum value to the underlying type

Hello

This source:

static_cast<std::underlying_type_t<T>>(value);

is compiled to the cvttsd2si instruction and hangs during debugging in latest version of MSVS 2019.

Details

This problem on godbolt: https://godbolt.org/z/jfrYv9zc8

I used MSVC of version 19.29.30148. It is from the latest version of MSVS 2019 for now. I compiled following code with options: c++20, x64, debug

#include <type_traits>
#include <format>

enum class Enum
{
    A = 1
};

template <class T> concept is_enum = std::is_enum_v<T>;

template <is_enum T>
struct std::formatter<T> : std::formatter<std::underlying_type_t<T>>
{
	constexpr auto format(T value, auto& format_context)
	{
        // Invalid generated code
		const auto undVal = static_cast<std::underlying_type_t<T>>(value);
		return formatter<std::underlying_type_t<T>>::format(undVal, format_context);
	}
};


template<typename T>
auto fn(T val)
{
    // The same cast, but all is ok
    auto v = static_cast<std::underlying_type_t<T>>(val);
    return v;
}


void main()
{
    std::format("a = {}", Enum::A);
    auto a = fn(Enum::A);
}

Assembler of casting in fn function just copy the value. But the casting in std::formatter::format uses cvttsd2si. Firstly it is incorrect for this action. Secondly my debugger hangs on this instruction.

MSVC of version 19.35 (on godbolt) compiles this code correctly.

The original problem is caused by enum formatter for fmt-library (not std). Original code was for fmt-library (not std) and can be compiled with c++17. It is also compiled correctly with my compiler (MSVC 19.29).

Migration to MSVS 2022 is imposible. Fix it in MSVS 2019 please.

Thanks in advance.