External constant in header?

Question

C++ / C#

Why is it not possible to use extern const in headers if the header itself is not included in the .cpp file?
For example:
MyVar.h
extern const int AnyVar ;
MyVar.cpp
const int AnyVar = 475 ;
main.cpp

#include"MyVar.h"
int main()
{
     int test = AnyVar ; // ошибка
}

But if I fix MyVar.cpp by adding #include"MyVar.h", then everything works fine.
Moreover, if an external non-constant variable is declared in the header, then you don’t have to do this (connect the header to the .cpp implementation).
Why does this happen with constants?

Answer 1

Maybe it will help