Why a static const variable assigned from offsetof gets an error if it is not marked inline?

Question

For some reason, this code runs (!) but with the raised error if I do not add inline. And works fine, otherwise. Why so?

#include <iostream>

#define NAME b

template <typename T>
struct B {
  static const ptrdiff_t shift = offsetof(T, NAME);  // it runs (!) but gives an error
  // inline static const ptrdiff_t shift = offsetof(T, NAME);  // works fine
};

struct A {
  B<A> b;
};

int main() {
  return 0; 
}

offsetof is underlined with a red squiggly line:

Offsetof of incomplete type 'A'clang(offsetof_incomplete_type)