instantiates some ctype templates which have not yet been defined

Right. And the standard only requires ctype specializations for char and wchar_t
(see table 80)

Actually, it looks like much of the streaming infrastructure depends on having a valid ctype available; so this fails, too:

#include
#include
#include
#include

template <class CharT, class Traits = std::char_traits>
std::basic_string<CharT, Traits> stringViaStream ( const CharT *p ) {
std::basic_stringstream<CharT, Traits> ss;
std::basic_string<CharT, Traits> s;

ss << p;
ss >> s;
assert ( s == p );
return s;
}

int main () {
stringViaStream ( "123");
stringViaStream (L"123");
// stringViaStream (U"123");
// stringViaStream (u"123");
}

Or rather, it will fail if you uncomment either of the last two lines.

That's now Table 90 (http://wg21.link/locale.category).
Closing as "won't fix".