是否保证永远不会发生类似函数的宏参数名称替换？

Question

Consider the following code

#define foo 38
#define F(foo) G(foo)
F(42);

I expect this code to fail to compile because after applying the first line second line should transform into #define F(38) G(38) which does not make any sense. But on g++ it successfully compiles into G(42) , as if there was no first line at all. I was unable to find any mention of this behaviour in neither g++ docs nor c standard. I know that code is ugly and should not be used in the first place, but I wonder if it has any guarantees to be portable.

Answer 1

C 2018 6.10 7 and C++ 2017 (draft n4659) 19 [cpp] 6 both say:

The preprocessing tokens within a preprocessing directive are not subject to macro expansion unless otherwise stated.

For parameters of function-like macros, nothing is otherwise stated. Therefore, the parameter names in the definition of a function-like macro are not replaced due to prior macro definitions.

(Examples of where macro expansion is otherwise stated include #if directives and #include directives.)