[Bug libstdc++/88508] New: std::bad_cast in std::basic_ostringstream<char16_t>.oss.fill()

[dudin.roman at list dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88508

            Bug ID: 88508
           Summary: std::bad_cast in
                    std::basic_ostringstream<char16_t>.oss.fill()
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dudin.roman at list dot ru
  Target Milestone: ---

Created attachment 45241
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=45241&action=edit
Code

I am trying to add support of UTF-16 and UTF-32 in C++17 environment.

Short fragment of my code:

namespace std {
    typedef basic_ostringstream<char16_t> u16ostringstream;
}

std::u16string to_hexdump_u16(const std::u16string_view &str_v) {

    std::u16ostringstream oss;
    oss.setf(std::ios::hex, std::ios::basefield);
    oss.fill(u'0');

...

    return oss.str();
}

int main() {

    auto str_u16 = u"Hello, my favorite World with native UTF-16 & UTF-32
support!♥❤♡";
    auto str_dump_u16 = to_hexdump_u16(str_u16);

    return 0;
}

I compile it via CMake with set(CMAKE_CXX_STANDARD 17)
It compiles successfully, but the bad_cast exception occurs when
"oss.fill(u'0')" is called. The internal function which throws the exception is
"__check_facet(const _Facet* __f)" in basic_ios.h.

Exception is also thrown when calling "oss << std::endl", but I think it's the
same problem.

Problem touches latest library compiled with clang (linux), gcc 8.1 (mingw-w64,
linux). The code compiles and works fine on latest MSVC compiler. 

The one-file full example in attach.