I'm assuming you meant SomeFunction().

SomeFunction could return an Lvalue, a PRvalue, or an Xvalue, for example:

    mutex   SomeFunction(void);

    mutex  &SomeFunction(void);

    mutex &&SomeFunction(void);

This is why we need:

    decltype(SomeFunction) x = SomeFunction();

    if ( x ) return !x;

And so the problem is that you need to type out the expression twice, give the variable a non-clashing name, and mention the variable's name twice as well.

Much easier to mention the function only once and to not mention the variable at all.