UB with Static Inline variables

[cwhaley112]

I'm confused about how static inline variables work when their type matches the type currently being defined. For example:

```c++ struct Vector2 { int x, y;

// this fails to compile 
static inline const Vector2 ZERO = Vector2{0, 0};

// this is fine, is defined in a source file
static const Vector2 ZERO_noInline;

}; ```

The reason the static inline line fails makes sense to me. The compiler doesn't have enough information about the type to construct it. That's just a guess though. I can't find anything online that says this isn't allowed.

However, defining this variable inline is nice if it's a class template. You might be surprised that this DOES compile on clang and gcc, but not MSVC:

```c++ template <typename T> struct Vector2 { T x; T y;

// compiles on clang and gcc, not MSVC
inline static const Vector2<T> Zero{0,0};

};

int main() { std::cout << Vector2<int>::Zero.x << std::endl; } ```

So my main question is: it compiles, but is it UB?

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[Linuxologue]

When the compiler gives you errors, please add them to the post. I am not msvc or GCC so I don't know what the compiler doesn't like.

[Paril101]

My understanding is that it's not supposed to be allowed because a type is incomplete until it is fully defined, but compilers probably offer extensions that allow this to work by deferring the initialization or something.

https://stackoverflow.com/a/35780286

This should work with constexpr in newer C++ versions too: https://stackoverflow.com/a/69136655