Question

Minimal code:

// --------inline.h--------
struct X { 
  static inline void foo ();
};   
#ifdef YES
inline void X::foo () { cout << "YES\n"; }
#else
inline void X::foo () { cout << "NO\n"; }
#endif

// --------file1.cpp--------
#define YES    // <---- 
#include"inline.h"
void fun1 ()
{
  X::foo();
}

// --------file2.cpp--------
#include"inline.h"
void fun2 ()
{
  X::foo();
}

If we call fun1() and fun2(), then they will print YES and NO respectively, which means they are referring different function bodies of same X::foo().

Irrespective of this should be coded or not, my question is:
Is this a well defined or undefined behavior ?

Answer 1

Yes it is Undefined Behavior.

Reference:

C++03 Standard:

7.1.2 Function specifiers [dcl.fct.spec]
Para 4:

An inline function shall be defined in every translation unit in which it is used and shall have exactly the same definition in every case (3.2). [Note: a call to the inline function may be encountered before its definition appears in the translation unit. ] If a function with external linkage is declared inline in one translation unit, it shall be declared inline in all translation units in which it appears; no diagnostic is required. An
inline function with external linkage shall have the same address in all translation units. A static local variable in an extern inline function always refers to the same object. A string literal in an extern inline function is the same object in different translation units.

Note: 3.2 refers to the One Definition Rule, Which states:

3.2 One definition rule [basic.def.odr]
Para 1:

No translation unit shall contain more than one definition of any variable, function, class type, enumeration type or template.