Incomplete class type when using various members of <iterator>

The following code sample results in an incomplete type error: #include <iterator> int main(void) {         bool always_false = false;         if (always_false) {                 std::istreambuf_iterator<char> cl;                 std::istreambuf_iterator<char> var1;                 const std::istreambuf_iterator<char> & arg1 = var1;                 (void) cl.equal(arg1);         }         return 1; } The question is whether including <iterator> ought to include <streambuf>, which includes the definition for std::basic_streambuf, which is required by the definition of std::istreambuf_iterator.