IsChildOf/IsParentOf return false for every path directly under a filesystem root ("/home" vs "/", "C:\foo" vs "C:\")

[matt-edmondson]

What's wrong

The span-based IsChildOf in Semantics.Paths/Implementations/AbsoluteDirectoryPath.cs (~lines 218-228) checks for a prefix match. It then requires a directory separator at thisPath[parent.Length]. IsParentOf delegates to the same logic, and AbsoluteFilePath.IsChildOf has an equivalent check.

A filesystem root already ends in a separator (/, C:\). For a path directly under a root, the character at parent.Length is therefore the first letter of the child's name, not a separator, so the check fails.

Reproduction

(Linux; the same happens on Windows with C:\foo and C:\.)

var root = AbsoluteDirectoryPath.Create<AbsoluteDirectoryPath>("/");
var home = AbsoluteDirectoryPath.Create<AbsoluteDirectoryPath>("/home");
home.IsChildOf(root);   // False, expected True
root.IsParentOf(home);  // False, expected True
AbsoluteFilePath.Create<AbsoluteFilePath>("/etc.txt").IsChildOf(root); // False, expected True

The type contradicts itself: home.Parent returns /, and GetAncestors() yields /. Deeper paths such as /home/user under /home work correctly.

Why it matters

Containment checks are a typical guard, for example "is this file inside the chosen directory?". They give the wrong answer whenever the user picks a drive or filesystem root, which is a common choice.

Suggested fix / acceptance criteria

• After the prefix match, return true when the parent ends in a separator (Path.EndsInDirectorySeparator(parentPathSpan)) and the child is longer. Otherwise keep the existing next-character check.
• Apply the change to both AbsoluteDirectoryPath and AbsoluteFilePath.
• Add tests for /home vs /, /etc.txt vs /, and (Windows-conditional) C:\foo vs C:\. Add a negative test that /home2 is not a child of /home.

Confirmed by running against main (8c06aba) on Linux.

[matt-edmondson]

Triage

• Category: Bug
• Priority: Medium. IsChildOf/IsParentOf return false for every path directly under a filesystem root, which contradicts Parent and GetAncestors(). The error fails closed: containment guards reject rather than admit. Deeper paths work.
• Area / suggested owner: Semantics.Paths (AbsoluteDirectoryPath.IsChildOf and the equivalent check in AbsoluteFilePath)
• Duplicates / in progress: None found. No open PR.

Next step: After the prefix match, accept the path when the parent already ends in a separator. Apply this to both types, and add the root, file-under-root and Windows-conditional tests, plus the /home2-is-not-under-/home negative test.

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