Link error with MSVC but not with g++ with constexpr - c++

Question

Consider the following code:
#include <iostream>
struct FactoryTag
{
static struct Shape {} shape;
static struct Color {} color;
};
template <typename TFactory>
int factoryProducer(TFactory tag)
{
if constexpr (std::is_same<TFactory, FactoryTag::Shape>::value)
return 12;
else if constexpr (std::is_same<TFactory, FactoryTag::Color>::value)
return 1337;
}
int main()
{
std::cout << factoryProducer(FactoryTag::shape) << std::endl;
return 0;
}
It works fine with g++ -std=c++1z Main.cpp but in Visual Studio with MSVC set with c++17 support it gives
Error LNK2001 unresolved external symbol "public: static struct FactoryTag::Shape FactoryTag::shape" (?shape#FactoryTag##2UShape#1#A) StaticTest C:\Users\danielj\source\repos\StaticTest\StaticTest\StaticTest.obj 1
Is this a bug in MSVC?

Answer 1

Is this a bug in MSVC?
No, FactoryTag::shape is odr-used here, so it needs a definition (you're copy-constructing it, which goes through the implicitly generated copy constructor, which requires you to bind a reference). Nor is this a bug in gcc either, arguably, since there is no diagnostic required if a definition is missing.
The solution is to add a definition. The old way would be:
struct FactoryTag { ... };
Shape FactoryTag::shape{}; // somewhere in a source file
The new way would be:
struct FactoryTag {
struct Shape {} static constexpr shape {}; // implicitly inline in C++17
};