isAbsolute(r'\') should return false for Windows-style filesystem

[aiurovet]

As per comment by @jamesderlin at https://github.com/google/file.dart/issues/198#issuecomment-1518274694 on the issue related to the file package,

The expectation is that _context.isAbsolute(path) logically implies that path includes style.drive as a prefix, which isn't true in this case. Seems like the fix for that might be more appropriate for package:path, but I'm reluctant to touch that because I'm not sure what changes are planned (or when) for v2.0 of that package.

[lrhn]

From the documentation of isAbsolute on Windows, returning false for r"\" is the correct and desired behavior.

It's doesn't do that. Tested on DartPad:

import "package:path/path.dart" as path;
void main() {
  var c = path.Context(style: path.Style.windows);
  print(c.isAbsolute(r'')); // false
  print(c.isAbsolute(r'\')); // true ??
  print(c.isAbsolute(r'\\')); // true
  print(c.isAbsolute(r'a:')); // false
  print(c.isAbsolute(r'a:\')); // true
}

I think the bug is in this line:
https://github.com/dart-lang/path/blob/master/lib/src/style/windows.dart#L51

    if (path.length < 2 || path.codeUnitAt(1) != chars.backslash) return 1;

where it should return 0 as "root length" for a single \ input, so:

    if (path.length < 2) return 0;
    if (path.codeUnitAt(1) != chars.backslash) return 1;

[jamesderlin]

From the documentation of isAbsolute on Windows, returning false for r"\" is the correct and desired behavior.

Yes. When I wrote "The expectation is that _context.isAbsolute(path) logically implies that path includes style.drive as a prefix, which isn't true in this case", I meant that the implication (the boolean logic operation) wasn't true, not that _context.isAbsolute(path) wasn't returning true. _context.isAbsolute(r'\') currently returns true but should be returning false. Sorry for not being clearer.

[aiurovet]

OK, I'm happy to close the issue.

[jamesderlin]

No no, isAbsolute is returning the wrong value, which is definitely a bug.