Preprocessor: incorrect expansion with whitespace + paste operator

[ehaas]

#define CONTINUE(k) DEFER(k##_HOOK)()
#define DEFER(op) op EMPTY
#define EMPTY
#define F_HOOK() F

CONTINUE( F)
CONTINUE(F)

gcc and clang both produce:

F_HOOK ()
F_HOOK ()

aro produces:

 F
F_HOOK ()

The behavior was the same before the hideset change so I don't think it's related to that.

[ehaas]

Replacing empty object macros with a placemarker token "fixes" this but I'm not sure why the space breaks it in the first place.

[ehaas]

Smaller example of the paste operator misbehaving:

#define F(X) (X##Y)

F( )

If there is whitespace inside the parentheses, it errors with:

test.c:1:14: error: pasting formed '(Y', an invalid preprocessing token
#define F(X) (X##Y)
             ^

With no whitespace is correctly expands to (Y)