How should `..` paths be resolved?

[sunfishcode]

On POSIX, path resolution is performed one component at a time, and .. is resolved only after resolving all components that came before it. Opening a path such as nonexistent/.. fails, because opening nonexistent first fails.

On Windows, path resolution has an up-front parsing step which resolves ... On nonexistent/.., it's resolved to . first, so opening it succeeds.

Should we require Windows implementations to emulate POSIX semantics here, or say that whether .. is resolved up front or on the fly is nondeterministic?