<fstream>: basic_filebuf doesn't comply with setbuf(0,0) requirement in the standard

[filebuf.virtuals]/12 says:

If setbuf(0, 0) is called on a stream before any I/O has occurred on that stream, the stream becomes unbuffered. Otherwise the results are implementation-defined. "Unbuffered" means that pbase() and pptr() always return null and output to the file should appear as soon as possible.

We don't accept pubsetbuf() before a file has been opened. libstdc++ accepts pubsetbuf() only before a file has been opened. Opening the file should probably be considered I/O. Note that even if you move pubsetbuf() after the open we still don't do the right thing.

Test case

Based on libcxx/test/std/input.output/file.streams/fstreams/filebuf.virtuals/overflow.pass.cpp:

1. >type repro.cpp
#include <fstream>
#include <iostream>

template <class CharT>
struct test_buf
    : public std::basic_filebuf<CharT>
{
    typedef std::basic_filebuf<CharT>  base;
    typedef typename base::char_type   char_type;
    typedef typename base::int_type    int_type;
    typedef typename base::traits_type traits_type;

    char_type* pbase() const {return base::pbase();}
    char_type* pptr()  const {return base::pptr();}
    char_type* epptr() const {return base::epptr();}
    void gbump(int n) {base::gbump(n);}

    virtual int_type overflow(int_type c = traits_type::eof()) {return base::overflow(c);}
};

#define ASSERT_OR_FAIL(x) {num++; if (!(x)) { std::cout << "FAILED at #" << num << std::endl; return -1; } }

int main()
{
    int num = 0;
    test_buf<char> f;
    f.pubsetbuf(0, 0);
    // #1
    ASSERT_OR_FAIL(f.open("overflow.dat", std::ios_base::out) != 0);
    // #2
    ASSERT_OR_FAIL(f.is_open());
    // #3
    ASSERT_OR_FAIL(f.pbase() == 0);
    // #4
    ASSERT_OR_FAIL(f.pptr() == 0);
    // #5
    ASSERT_OR_FAIL(f.epptr() == 0);
    // #6
    ASSERT_OR_FAIL(f.overflow('a') == 'a');
    // #7
    ASSERT_OR_FAIL(f.pbase() == 0);
    // #8
    ASSERT_OR_FAIL(f.pptr() == 0);
    // #9
    ASSERT_OR_FAIL(f.epptr() == 0);
}

2. >cl /EHsc /W4 /WX .\repro.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.27.29009.1 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

repro.cpp
Microsoft (R) Incremental Linker Version 14.27.29009.1
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:repro.exe
repro.obj

3. >.\repro.exe
FAILED at #7

Expected behavior
The tests pass

STL version

Microsoft Visual Studio Community 2019 Preview
Version 16.7.0 Preview 3.0

Additional context

• This must be an ABI breaking change as it forces us to remember whether that call happened somewhere.

• Skipped libcxx test

https://github.com/microsoft/STL/blob/faccf0084ed9b8b58df103358174537233b178c7/tests/libcxx/expected_results.txt#L407-L409

• Also tracked by Microsoft-internal AB#595631

Start with the standalone repro.cpp and compare its assertions with the referenced libcxx overflow.pass.cpp test. Review tests/libcxx/expected_results.txt around the skipped entry, then locate the basic_filebuf implementation and its ABI constraints. Done means the unbuffered setbuf(0, 0) behavior satisfies all listed assertions and the skipped test can be enabled.