`<filesystem>`: `read_symlink` does not work with junctions

[SibiSiddharthan]

Command-line test case

C:\Temp> mkdir a
C:\Temp> mklink /J b a

C:\Temp>type repro.cpp
#include <filesystem>
#include <iostream>

using namespace std;
namespace fs = std::filesystem;

int main()
{
	error_code ec;
	auto p = fs::read_symlink("b",ec);
	printf("%ls %d %s\n",p.c_str(), ec.value(), ec.message().c_str());
}

C:\Temp>cl /EHsc /std:c++17 .\repro.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.23.28019.1 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

repro.cpp
Microsoft (R) Incremental Linker Version 14.23.28019.1
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:repro.exe
repro.obj

C:\Temp>.\repro.exe

Expected behavior
a 0 The operation completed successfully.

Observed behavior
 4393 The tag present in the reparse point buffer is invalid.

STL version

9a9820d

[SibiSiddharthan]

I tried debugging this and here are my findings.

When we do DeviceIoControl with FSCTL_GET_REPARSE_POINT, if the HANDLE refers to a symlinked file the ReparseTag reads
2684354572 which is IO_REPARSE_TAG_SYMLINK (0xA000000C). But if the HANDLE points to junction the ReparseTag reads
2684354563 which is IO_REPARSE_TAG_SYMLINK - 9. I am not sure why this happens.

Another weird thing

If the HANDLE refers to a symlinked file, the SymbolicLinkReparseBuffer.Flags reads 1 which is SYMLINK_FLAG_RELATIVE 0x1 and B->SymbolicLinkReparseBuffer.PathBuffer reads the relative path wrt the current working directory. This is as expected. But
when the HANDLE refers to a junction , the SymbolicLinkReparseBuffer.Flags reads 4128860 ? and B->SymbolicLinkReparseBuffer.PathBuffer reads the absolute path excluding the volume. Is this expected behavior or am I missing something?

[SibiSiddharthan]

I got confused between IO_REPARSE_TAG_MOUNT_POINT and IO_REPARSE_TAG_SYMLINK. Things work as expected. Sorry about this. Closing this issue.