Why does this compile: string = int

Suppose the following code:

#include <iostream>
#include <string>

int func() { return 2; }

int main()
{
   std::string str("str");
   str = func();
   std::cout << "Acquired value: '" << str << "'" << std::endl;

   return 0;
}

Why does the line str = func(); compile without warning of a type mismatch?

I'm using compiler gcc v. 4.7.1 with the -std=c++11 flag set.

Output:

Acquired value: ''