https://issues.dlang.org/show_bug.cgi?id=22059

          Issue ID: 22059
           Summary: std.file: Long paths support on Windows requires
                    explicit \\?\ prefix
           Product: D
           Version: D2
          Hardware: All
                OS: Windows
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody@puremagic.com
          Reporter: kinke@gmx.net

```
void main()
{
    import std.file;
    // works

assert(exists(`\\?\C:\Users\Martin\AppData\Local\dub\packages\mir-algorithm-1.0.1\mir-algorithm\.dub\build\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D.obj`));
    // fails

assert(exists(`C:\Users\Martin\AppData\Local\dub\packages\mir-algorithm-1.0.1\mir-algorithm\.dub\build\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D.obj`));
}
```

The prefix shouldn't be required and added automatically, as the Visual Studio 2019 linker doesn't require that prefix, nor does the LLD linker, nor does DMD, ...

Phobos should essentially use something very similar to DMD's
extendedPathThen() helper:
https://github.com/dlang/dmd/blob/ef39b070819216a8d9835e2a66d3e365420763ec/src/dmd/root/filename.d#L1146

--