Describe the bug
C++ Standard, [filebuf], paragraph 2 states that:

The restrictions on reading and writing a sequence controlled by an object of class basic_filebuf<charT, traits> are the same as for reading and writing with the C standard library FILEs

So when referring to C standard §7.19.5.3, paragraph 6 states that:

However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file

One could therefore expect that for a bidirectional std::fstream you could perform stream.seekp(0, std::ios::cur) to satisfy this. In libstdc++ and libc++, this works but this does not work with MSVC STL.

Note that stream.seekp(stream.tellp()) works. So does a +1 and -1 on std::ios:cur.

Command-line test case

#include <fstream>
#include <iostream>

int main() {
	std::ofstream init("test.txt");
	init << "abcd";
	init.close();

	char buffer[5] = {};

	std::fstream inout("test.txt");
	inout.read(buffer, 2);
	inout.seekp(0, std::ios::cur); // Should allow switch to write.
	inout << "ba";
	inout.close();

	std::ifstream out("test.txt");
	out.read(buffer, 4);
	std::cout << buffer << std::endl;
	out.close();
}

Expected behavior
The output is "abba".

MSVC STL outputs "abcd" - the write operation is completely ignored.

STL version

Microsoft Visual Studio Enterprise 2022
Version 17.5.2

Additional context

C++ standard, [filebuf.virtuals], paragraph 13 states:

Let width denote a_codecvt.encoding(). If is_open() == false, or off != 0 && width <= 0, then the positioning operation fails. Otherwise, if way != basic_ios::cur or off != 0, and if the last operation was output, then update the output sequence and write any unshift sequence. Next, seek to the new position: if width > 0, call fseek(file, width * off, whence), otherwise call fseek(file, 0, whence)

If I've not misread this: although stream.seekp(0, std::ios::cur) should not touch the output/unshift sequence, it still should call fseek to my understanding.