The preprocessor doesn't inhibit expansion when concatenating with an empty token.
Here an example:

#define EMPTY()
#define DEFER(id) id EMPTY()

#define A(p, ...) p ## __VA_ARGS__ // should inhibit expansion of ...
                                   
#define B(p, ...) p __VA_ARGS__

#define TEST() 123

A(, DEFER(TEST)()) // should be TEST (), but clang expands to 123
B(, DEFER(TEST)()) // should be 123