Why does warning "forcing value to bool 'true' or 'false'" disappear when using not operator?

Consider the following code compiled with Visual Studio 2015:

#include <iostream>
#include <cassert>

void foo(bool b)
{
    std::cout << b;
}

int main()
{
    int a;

    foo(a = 2);       // Getting warning #4800
    foo(!(a = 2));    // Not getting any warning

    return 0;
}

foo(a = 2) produces warning 4800 'int': forcing value to bool 'true' or 'false', fine.

But foo(!(a = 2)) does not produce the warning. Why? At some point there has been an int-to-bool cast!

foo(a = 2) is equivalent to bool b = (a = 2). The expression a = 2 returns an a, so it is equivalent to

a = 2;
bool b = a; //Conversion of 'int' to 'bool' -> Warning!

foo(!(a = 2)) is equivalent to bool b = !(a = 2). The expression a = 2 returns a:

a = 2;
bool b = !a; //'!a' is legal => It returns a bool -> No warning!

Note that you can apply operator! to an int, which negates the int, and so returns a bool. That's why there is no performance warning.