c - UNUSED macro warning

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So I found this href="https://stackoverflow.com/questions/4851075/universally-compiler-independent-way-of-implementing-an-unused-macro-in-c-c/4851173#4851173">macro
on SO:

#define
            UNUSED(x) (void)(sizeof((x),
            0))

and this (still)
produces the following warning:

main.c:11:36:
warning: left-hand operand of comma expression has no effect [-Wunused-value]

#define UNUSED(x) (void)(sizeof((x),
0))

Whereas
the simpler version, a normal void cast: #define
UNUSED(x) (void)(x) is
warning-free.

What could be the reason behind
it? In general warnings are a sign of high-risk situations. Is here the given warning
really useful?

I am interested in
C-explanation.

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class="normal">Answer

This macro
seems inappropriate for your compiler at the current warning
level.

You could use this simpler
version:

#define
            UNUSED(x)
            (void)(sizeof(x))

x
will not be evaluated either but is used so the compiler should not
complain about x being unused, not about the left hand side of
the , operator being unused in the
expression.