fix: show require() error on unsupported directory import

lib/nodejs/esm-utils.cjs

@@ -116,6 +116,13 @@ const requireModule = async (file, esmDecorator) => {
         throw requireErr;
       }
 
+      // A directory specifier resolves under CommonJS but not under ESM so this error
+      // only means the fallback import() never got far enough to say anything useful about the file.
+      // Check this issue https://github.com/mochajs/mocha/issues/5792
+      if (importErr.code === "ERR_UNSUPPORTED_DIR_IMPORT") {
+        throw requireErr;
+      }
+
       // Similarly, for an exports/imports mismatch such as a missing 'default',
       // the require() error will be more informative for users.
       // See: https://github.com/mochajs/mocha/issues/5411

test/node-unit/esm-utils.spec.cjs

@@ -6,6 +6,10 @@ const url = require("node:url");
 
 describe("esm-utils", function () {
   describe("requireOrImport", function () {
+    afterEach(function () {
+      sinon.restore();
+    });
+
     it("should show an informative error message for a broken default import", async function () {
       return expect(
         () =>
@@ -62,6 +66,29 @@ describe("esm-utils", function () {
         },
       );
     });
+
+    it("should surface the require() error when import() fails on a directory specifier", async function () {
+      sinon
+        .stub(esmUtils, "doImport")
+        .rejects(
+          Object.assign(
+            new Error("Directory import '/fixtures/lib' is not supported"),
+            { code: "ERR_UNSUPPORTED_DIR_IMPORT" },
+          ),
+        );
+
+      return expect(
+        () =>
+          esmUtils.requireOrImport(
+            "../../test/node-unit/fixtures/this-module-does-not-exist.cjs",
+          ),
+        "to be rejected with error satisfying",
+        {
+          code: "MODULE_NOT_FOUND",
+          message: /this-module-does-not-exist/,
+        },
+      );
+    });
   });
 
   describe("loadFilesAsync()", function () {