C++: implicit cast of stream to bool

Take this toy code:

#include <iostream>
#include <fstream>

int main() {

    std::ifstream is;
    \\ perform read
    \\ ...
    if (!is) // works
        std::cout << "fail";
    if( is == false)  // error C2678
        std::cout << "fail";

    return 0;
}

You'd get the following counter-intuitive results: if(is) compiles, and if(is==true) gives

error C2678: binary '==': no operator found which takes a left-hand operand of type 'std::ifstream' (or there is no acceptable conversion)

(for VS2015 - similar errors in gcc and clang).

The standard says (according to this answer):

Valid C++ 2003 code that relies on implicit boolean conversions will fail to compile with this International Standard. Such conversions occur in the following conditions:

• passing a value to a function that takes an argument of type bool;

• using operator== to compare to false or true;

• returning a value from a function with a return type of bool;

• initializing members of type bool via aggregate initialization;

• initializing a const bool& which would bind to a temporary.

As far as I can tell if(is==false) is explicitly required to fail, but how come if(!is) doesn't? Doesn't it qualify as 'implicit boolean conversion'?