C++, variable declaration in 'if' expression

As of C++17 what you were trying to do is finally possible:

if (int a = Func1(), b = Func2(); a && b)
{
    // Do stuff with a and b.
}

Note the use of ; of instead of , to separate the declaration and the actual condition.

I think you already hinted at the issue. What should the compiler do with this code?

if (!((1 == 0) && (bool a = false))) {
    // what is "a" initialized to?

The "&&" operator is a short-circuit logical AND. That means that if the first part (1==0) turns out to be false, then the second part (bool a = false) should be not be evaluated because it is already known that the final answer will be false. If (bool a = false) isn't evaluated, then what to do with code later on that uses a? Would we just not initialize the variable and leave it undefined? Would we initialize it to the default? What if the data type was a class and doing this had undesirable side effects? What if instead of bool you used a class and it had no default constructor such that the user must provide parameters - what do we do then?

Here's another example:

class Test {
public:
    // note that no default constructor is provided and user MUST
    // provide some value for parameter "p"
    Test(int p);
}

if (!((1 == 0) && (Test a = Test(5)))) {
    // now what do we do?!  what is "a" set to?

Seems like the limitation you have found seems perfectly reasonable - it prevents these kinds of ambiguities from happening.