msvc inline static member variable redefinition - c++

Question

I'm new to C++17. Considering the following code:
// ---------------
// in MyClass.hpp
// ---------------
#pragma once
class MyClass {
public:
static const int A;
};
inline const int MyClass::A = 100;
// ---------------
// in test.cpp
// ---------------
#include <stdio.h>
#include "MyClass.hpp"
void test() {
printf("test: %p\n", &MyClass::A);
}
// ---------------
// in main.cpp
// ---------------
#include <stdio.h>
#include "MyClass.hpp"
extern void test();
int main() {
printf("main: %p\n", &MyClass::A);
test();
}
When compiled with MinGW-W64 g++ 8.1.0
g++ -std=c++17 main.cpp test.cpp -o test.exe
The output is
main: 00000000004044B0
test: 00000000004044B0
which works as expected.
However, in MSVC 2017
cl /std:c++17 main.cpp test.cpp
I got a compiler error, saying redefinition of "public: static int const MyClass::A". (Sorry, the compiler output contains Chinese characters. It's not appropriate to post here directly.)
Why the code works under g++, but fails in MSVC? Did I do something wrong?

Answer 1

I can confirm that Clang accepts your code without any warning.
What worries me is that cppreference shows the following note:
The inline specifier cannot re-declare a function or variable (since C++17) that was already defined in the translation unit as non-inline.
I could not really identify the real cause for that note in C++ standard. But as cppreference is generally correct in its warnings, I assume that it is the reason why MSVC chokes on your code. It would probably expect:
// ---------------
// in MyClass.hpp
// ---------------
#pragma once
class MyClass {
public:
static const int A = 100;
};
in order to avoid a former non inline declaration followed with an inline definition.