C-style vs Function-style: Best Cast for the Unused Variable/Function Warning

When using a cast to void in order to suppress/silence a warning for an unused variable or function there are different possible combinations:

static int foo(){ return 4; }

int main(){
    int x;

//   void   x;      // Error: void variable
//   void  (x);     // Error: void variable
    (void)  x;      // OK
    (void) (x);     // OK

//   void   foo();  // Error: new function declaration
//   void  (foo()); // Error: new function declaration
    (void)  foo();  // OK
    (void) (foo()); // OK

//   void   foo;    // Error: void variable
//   void  (foo);   // Error: void variable
    (void)  foo;    // OK
    (void) (foo);   // OK

    return 0;
}

In order to be able to use the second combination (01), that is, function-style cast, I tried enclosing the expression in parentheses and it seems to be correct:

(void  (x));
(void  (foo()));
(void  (foo));

This strategy cannot make the first combination (00) work as a cast either, and I interpret it is not needed neither in the third nor in the fourth. Of course, even more extra meaningless parentheses could be added. Actually, if my understanding is correct, the fourth combination (11) contains indeed such unnecessary parentheses, and it is just the same C-style cast as the third combination (10).

My question is which of these approaches is the best, making sure it is not optimized out by the compiler, while understanding that no extra needless parentheses are used. I did not explore in detail the syntax expansion of this kind of expressions, so maybe I am missing something.

Note that I will not use this construct in any macro, but directly on a variable/function. Besides, I am not concerned about side effects on volatile variables.

Links:

• Is a C-style cast identical to a function-style cast?
• Universally compiler independent way of implementing an UNUSED macro in C/C++
• Will a “variableName;” C++ statement be a no-op at all times?
• How do I best silence a warning about unused variables?
• Suppress Compiler warning Function declared never referenced
• How can I hide “defined but not used” warnings in GCC?