Rule 6.2.1: non-inline constexpr variables in headers?

[cgpzs]

Hi, 

Consider the following example:

Code:

// foo.h
constexpr int kFoo = 123;

inline int foo(int x)
{
  return x * kFoo;
}

// tu1.cpp
#include "foo.h"
int tu1() { return foo(1); }

// tu2.cpp
#include "foo.h"
int tu2() { return foo(2); }

Does the function "foo" violate Rule 6.2.1 in this context, given that kFoo is not a C++17 inline variable? 
Or would it violate 6.2.1 only if kFoo were ODR-used (for example, passed by reference)?

Thanks!