[libc++][NFC] Move basic_ios extern instantiations into <ios>

[philnik]

Summary

basic_ios is defined in <ios>, so it seems weird that we declare the explicit instantiation for it i <streambuf>, which is technically unrelated.

[philnik]

Fix diff

[EricWF]

You need to describe the rational in the description.

[EricWF]

Wait a second.

Are you sure that moving the declaration before the definition of a ton of member functions is behavior preserving?

[EricWF]

Did some digging, and it seems this should be fine. LGTM again.

[ldionne]

LGTM too, thanks for explaining!

[philnik]

Try to fix CI

[philnik]

Rebased

libcxx/include/ios

 template <class _CharT, class _Traits>
 inline _LIBCPP_INLINE_VISIBILITY
 void
 basic_ios<_CharT, _Traits>::set_rdbuf(basic_streambuf<char_type, traits_type>* __sb)
 {
     ios_base::set_rdbuf(__sb);
 }
 
+extern template class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS basic_ios<char>;
+
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+extern template class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS basic_ios<wchar_t>;
+#endif
+
 _LIBCPP_HIDE_FROM_ABI inline
 ios_base&
 boolalpha(ios_base& __str)
 {
     __str.setf(ios_base::boolalpha);
     return __str;
 }
 

libcxx/include/streambuf

 template <class _CharT, class _Traits>
 typename basic_streambuf<_CharT, _Traits>::int_type
 basic_streambuf<_CharT, _Traits>::overflow(int_type)
 {
     return traits_type::eof();
 }
 
 extern template class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS basic_streambuf<char>;
-extern template class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS basic_ios<char>;
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 extern template class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS basic_streambuf<wchar_t>;
-extern template class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS basic_ios<wchar_t>;
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP_STREAMBUF