Why/how is it valid to pass a non-wide string to wstringstream::operator<<?

Question

wstring ss;
ss << L"Some wide-char text" << " and some non-wide-char text";

This appears to work just fine but why, when char_type is not char?

Answer 1

Because it is.

Exactly std::char_traits<char>::length(s) characters are inserted.
Before insertion, first, all characters are widened using os.widen().

(source)

Answer 2

All streams have overloads for char, regardless of the CharT of the template.

http://en.cppreference.com/w/cpp/io/basic_ostream/operator_ltlt2

template< class CharT, class Traits >
basic_ostream<CharT,Traits>& operator<<( basic_ostream<CharT,Traits>& os, 
                                         const char* s );