The library does not provide an definition of std::operator== for arguments of type std::istream_iterator. That is, the following code is erroneously marked as ill-formed with a diagnostic: #include <iterator> #include <string> using ISTREAM_ITER_C = std::istream_iterator<char, char>; bool foo(const ISTREAM_ITER_C& arg00 ,const ISTREAM_ITER_C& arg01 ){ return std::operator== (arg00, arg01); // Syntax error } However, this is only an issue if called in exactly that way. Using it as an operator, rather than as a function call, works as expected: arg00 == arg01 // Works