This heuristic makes me a bit nervous. We've had issues before doing stuff like this - the isDir check is doing a stat, so requires all the paths to exist to behave correctly. In this instance, that might be ok, but I'm not sure it holds for the general case and might result in surprises for future callers of Dir().

In practice, while a pain, I think it'd be better to treat this more like the Go stdlib and ALWAYS return the enclosing dir and make the Abs function public (could rename it something like LoadedPath()?). That is, the caller needs to know what type of path they're carrying around (file or dir) and invoke the right function.

Can be checked running compose-spec command with --no-path-resolution

by the way, I'm confused why such a distinction is made. Dir should return owner directory, whenever path is a file or not

depending how you declare your path foo/bar or foor/bar/, filepath.Dir won't return the same output, foo in the first case and foo/bar in the second one
https://pkg.go.dev/path/filepath#example-Dir

then AFAICT we could just rely on filepath.Clean so that we don't need to check actual file exists but rely on file path representation