fix: ensure defaults are set in @babel/eslint-parser

eslint/babel-eslint-parser/src/configuration.js

@@ -9,7 +9,19 @@ export function normalizeESLintConfig(options) {
     requireConfigFile: true,
   };
 
-  return Object.assign(defaultOptions, options);
+  // ESLint sets ecmaVersion: undefined when ecmaVersion is not set in the config.
+  // Prune to ensure that defaults are respected.
+  const prunedOptions = Object.entries(options).reduce(
+    (options, [key, value]) => {
+      if (value !== undefined) {
+        options[key] = value;
+      }
+      return options;
+    },
+    {},
+  );
+
+  return Object.assign(defaultOptions, prunedOptions);
 }
 
 export function normalizeBabelParseConfig(options) {

eslint/babel-eslint-tests/test/integration/eslint/eslint.js

@@ -1,3 +1,5 @@
+import eslint from "eslint";
+import * as parser from "../../../../babel-eslint-parser";
 import verifyAndAssertMessages from "../../helpers/verifyAndAssertMessages";
 
 describe("verify", () => {
@@ -1879,3 +1881,23 @@ describe("verify", () => {
     );
   });
 });
+
+describe("ESLint config", () => {
+  it('should set ecmaVersion to latest and sourceType to "module" by default', () => {
+    const linter = new eslint.Linter();
+    linter.defineParser("@babel/eslint-parser", parser);
+
+    // ImportDeclarations result in a parser error if ecmaVersion < 2015 and sourceType != "module".
+    const messages = linter.verify('import { hello } from "greetings"', {
+      parser: "@babel/eslint-parser",
+      parserOptions: {
+        babelOptions: {
+          configFile: require.resolve(
+            "../../../../babel-eslint-shared-fixtures/config/babel.config.js",
+          ),
+        },
+      },
+    });
+    expect(messages.length).toEqual(0);
+  });
+});