fix(importer): reach package-main selection for trailing-slash directory references (#4219)

* fix(importer): reach package-main selection for trailing-slash directory references

posix.normalize preserves a trailing slash in the specifier, so normalized was sub/ while dirname(match) was sub; the resolvedThroughIndex comparison failed and selectPackageMain never ran for require("./sub/"). Compare against the slash-stripped directory so trailing-slash directory references take the same package-main path as the plain spelling. Boundary rows pair each trailing-slash request with the existing manifest fixtures and are validated against Node's own resolver in the same table.

Fixes #4189

* fix(importer): derive trailing-slash directory identity with path semantics

The lookbehind strip kept a trailing slash when the final component ended in a line terminator (require("./pkg\n/")), so dirname(match) still never matched and the trailing-slash path skipped package-main selection for those directories. Join the normalized path with "." instead: path semantics collapse any trailing slash run regardless of the preceding character, and behave identically to the strip on every other spelling. Boundary fixture gains a newline directory with a package.json, an index and a declared main so the index path (not the no-match branch) exercises the comparison.

Addresses Codex P1 on #4219

* fix(importer): normalize package-main targets with path semantics

---------

Co-authored-by: Hemang Varshney <218273197+hemv-857@users.noreply.github.com>
Co-authored-by: Kaoru0822 <208281322+kaoru0822-kitauji@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/application/ReferenceSourceImportEntries.ts

@@ -269,9 +269,9 @@ const selectPackageMain = (
   // package, and a trailing separator does not make the entry a directory.
   if (posix.isAbsolute(manifest.main))
     return { to: posix.normalize(manifest.main), resolution: "unresolved" };
-  const target = posix
-    .normalize(posix.join(packageDirectory, manifest.main))
-    .replace(/(?<=.)\/+$/u, "");
+  // Joining "." strips the trailing separator run with path semantics; a
+  // lookbehind regex kept separators behind line terminators (main "lib\n/").
+  const target = posix.join(posix.join(packageDirectory, manifest.main), ".");
   // Node would load a declared entry outside the imported root.
   if (logicalPathEscapesRoot(target))
     return { to: target, resolution: "unresolved" };
@@ -395,9 +395,15 @@ const resolveInternalSpecifier = (
     return packageMainForDirectory(normalized, context) !== undefined
       ? selectPackageMain(normalized, undefined, context, mode)
       : { to: normalized, resolution: "unresolved" };
+  // posix.normalize preserves the specifier's trailing slash while
+  // dirname(match) never carries one; derive the directory identity with path
+  // semantics (joining "." collapses trailing slashes, including ones a regex
+  // would keep behind a line terminator) so a trailing-slash reference still
+  // reaches package-main selection the same way the plain spelling does.
+  const directory = posix.join(normalized, ".");
   const resolvedThroughIndex =
     posix.basename(match).startsWith("index.") &&
-    posix.dirname(match).normalize("NFC") === normalized.normalize("NFC");
+    posix.dirname(match).normalize("NFC") === directory.normalize("NFC");
   return resolvedThroughIndex
     ? selectPackageMain(posix.dirname(match), match, context, mode)
     : { to: match, resolution: "internal" };

tests/boundary/cli/referenceSourceDirectoryImports.test.ts

@@ -37,10 +37,14 @@ const prepareReferenceSourceDirectoryFixture =
       ["legacy-order.cjs", "./dep", "dep/index.js"],
       ["bare-index.cjs", "./bare", "bare.js"],
       ["package-main.cjs", "./pkg", "pkg/actual.js"],
+      ["package-main-slash-specifier.cjs", "./pkg/", "pkg/actual.js"],
+      ["package-main-newline-slash.cjs", "./pkg\n/", "pkg\n/actual.js"],
       ["package-main-no-index.cjs", "./no-index-pkg", "no-index-pkg/actual.js"],
       ["package-main-fallback.cjs", "./stale", "stale/index.js"],
+      ["package-main-fallback-slash.cjs", "./stale/", "stale/index.js"],
       ["typed-index-collision.cjs", "./typed", "typed.js"],
       ["package-main-slash.cjs", "./slashed", "slashed/lib.js"],
+      ["package-main-manifest-newline-slash.cjs", "./nlpkg/", "nlpkg/lib\n.js"],
       ["json.cjs", "./data", "data.json"],
       ["json-explicit.cjs", "./data.json", "data.json"],
       ["json-before-index.cjs", "./conf", "conf.json"],
@@ -70,6 +74,12 @@ const prepareReferenceSourceDirectoryFixture =
       ["bare/index", 'module.exports = "extensionless index";\n'],
       ["only/index", 'module.exports = "extensionless index";\n'],
       ["pkg/package.json", '{"main":"actual.js"}\n'],
+      ["pkg\n/package.json", '{"main":"actual.js"}\n'],
+      [
+        "pkg\n/actual.js",
+        'module.exports = "declared main behind line terminator";\n',
+      ],
+      ["pkg\n/index.js", 'module.exports = "index not selected";\n'],
       ["pkg/actual.js", 'module.exports = "declared main";\n'],
       ["pkg/index.js", 'module.exports = "ignored index";\n'],
       ["no-index-pkg/package.json", '{"main":"actual.js"}\n'],
@@ -87,6 +97,13 @@ const prepareReferenceSourceDirectoryFixture =
       ["slashed/lib.js", 'module.exports = "file main";\n'],
       ["slashed/lib/index.js", 'module.exports = "not selected";\n'],
       ["slashed/index.js", 'module.exports = "not selected";\n'],
+      ["nlpkg/package.json", '{"main":"lib\\n/"}\n'],
+      [
+        "nlpkg/lib\n.js",
+        'module.exports = "file main behind line terminator";\n',
+      ],
+      ["nlpkg/lib\n/index.js", 'module.exports = "not selected";\n'],
+      ["nlpkg/index.js", 'module.exports = "entry index";\n'],
       ["absolute/package.json", `${JSON.stringify({ main: outside })}\n`],
       ["absolute/index.js", 'module.exports = "not selected";\n'],
       ["data.json", '{"value":1}\n'],