`<ostream>`: Exception from streambuf should be caught and not rethrown

[StephanTLavavej]

Originally reported as DevCom-10555755 / VSO-1939195 / AB#1939195 . I've cleaned up the repro and especially the Standardese citations, so I believe this is a bug.

D:\GitHub\STL\out\x64>type meow.cpp

#include <cassert>
#include <iostream>
#include <sstream>
#include <stdexcept>
using namespace std;

class MyStreamBuf : public streambuf {
protected:
    // N4971 [streambuf.virt.get]/1-2:
    // "streamsize showmanyc();
    // Returns: An estimate of the number of characters available in the sequence, or -1.
    // If it returns a positive value, then successive calls to underflow() will not return
    // traits::eof() until at least that number of characters have been extracted from the stream.
    // If showmanyc() returns -1, then calls to underflow() or uflow() will fail. [Note 277]
    // Default behavior: Returns zero."

    // Note 277:
    // "underflow or uflow can fail by throwing an exception prematurely. The intention
    // is not only that the calls will not return eof() but that they will return "immediately"."

    int_type underflow() override {
        throw runtime_error("WOOF");
    }
};

int main() {
    MyStreamBuf rdbuf;
    ostringstream oss;
    assert(oss.exceptions() == ios_base::goodbit);

    try {
        // N4971 [ostream.inserters]/8-9:
        // "basic_ostream& operator<<(basic_streambuf<charT, traits>* sb);
        // Gets characters from sb and inserts them in *this. Characters are
        // read from sb and inserted until any of the following occurs:
        // [...]
        // - an exception occurs while getting a character from sb.
        // If the function inserts no characters, it calls setstate(failbit) (which
        // may throw ios_base::failure ([iostate.flags])). If an exception was thrown
        // while extracting a character, the function sets failbit in the error state,
        // and if failbit is set in exceptions() the caught exception is rethrown."
        oss << &rdbuf;
        cout << "No exception observed in main().\n";
    } catch (const runtime_error&) {
        cout << "Caught runtime_error in main().\n";
    }

    assert(oss.rdstate() == ios_base::failbit);
}

D:\GitHub\STL\out\x64>cl /EHsc /nologo /W4 /MTd /Od meow.cpp && meow
meow.cpp
Caught runtime_error in main().

libstdc++ and libc++ print "No exception observed in main()." as shown by https://godbolt.org/z/Yq3noKn8s .