[Aryangupta-05]

memfs.ReadDir currently returns an empty result and no error for a regular file. Callers cannot distinguish that invalid directory path from an empty directory, unlike the OS-backed filesystem and the documented billy.Dir contract.

Return a *os.PathError wrapping syscall.ENOTDIR after resolving symlinks. The shared directory-conformance test compares the same call through memfs and osfs. A memfs regression also checks that a link to a file is rejected while a link to a directory remains readable.

Minimal reproduction:

fs := memfs.New()
f, err := fs.Create("file")
if err != nil {
    panic(err)
}
if err := f.Close(); err != nil {
    panic(err)
}
entries, err := fs.ReadDir("file")
fmt.Printf("entries=%v error=%v\n", entries, err)

Before: empty entries, nil error. After: nil entries and a non-nil path error. The OS-backed regression passed before the change; the memfs and symlink-to-file regressions failed before and pass after.

Validation on Windows / Go 1.26.8:

• go test ./memfs -count=1
• go test ./test -run '^TestDir_' -count=1
• go vet ./memfs ./test, gofmt and git diff --check
• Pinned golangci-lint 2.14.0: no newly introduced issues with --new-from-rev=eae82c9f2d37fb502d080eef0ab6bc1655501985.

Full-suite limitations were checked against a separate clean snapshot of the same upstream revision: both runs hit existing Windows symlink-privilege failures and the 45-second bound in the large TempFileMany stress test. Full lint reports unrelated findings in unchanged files on both snapshots; full vet flags the existing osfs.WriteTo signature. A full-suite pass or race-test pass is not claimed; the local C compiler could not start.

AI assistance: OpenAI Codex (GPT-6) assisted with investigation, implementation, regression tests, validation and this description. The commit includes an Assisted-by trailer and DCO sign-off.

[pjbgf]

@Aryangupta-05 thank you for working on this. 🙇