Writing "enum: int64_t" value to std::ostringstream truncates it to int

Question

This code behaves in an unexpected way with MSVC compiler (v141 toolset, /std:c++17):

#include <iostream>
#include <limits>
#include <sstream>
#include <stdint.h>

int main() {
    std::ostringstream ss;
    enum Enum : int64_t {muchos_digitos = std::numeric_limits<int64_t>::max() - 1000};
    ss << muchos_digitos;
    std::cout << ss.str();
    return 0;
}

Specifically, it prints "-1001". It is only after much head scratching and enabling /W4 warning level that I discovered the cause:

warning C4305: 'argument': truncation from 'main::Enum' to 'int'

But why does it happen? Indeed, the debugger confirms that int overload is called instead of long long, but why? And how can I circumvent this in generic code? I could cast muchos_digitos to int64_t, but I receive the value as typename T. I can figure out that it's an enum, but how can I know that it's a strongly typed enum, and can I find out its underlying type? I don't think it's directly possible...

The output is correct under GCC, but I need the code to work with all three of GCC, clang and MSVC.

Online demo

P. S. It was a mistake that /W4 was not set for my project in the first place. I recommend everyone to use this level with MSVC and -pedantic-errors with GCC / clang, it really saves you time with bizzare errors and surprising behavior when you notice it at compile time as you write the code.

Answer 1

This was confirmed to be a bug by the Microsoft team and it's now fixed in VS 2019: https://developercommunity.visualstudio.com/content/problem/475488/wrong-ostringstreamoperator-overload-selected-for.html