Repo: Integration test failure on main: flat-config-types-@types__eslint-v8 & -v9

[JoshuaKGoldberg]

Suggestion

Example from https://github.com/typescript-eslint/typescript-eslint/actions/runs/14020474109/job/39251785162?pr=10861:

FAIL  tests/flat-config-types-@types__eslint-v9.test.ts (54.762 s)
  ● flat-config-types-@types__eslint-v9 › typescript › should work successfully
    expect(received).toHaveLength(expected)
    Expected length: 0
    Received length: 5
    Received array:  ["eslint.config.js(55,19): error TS23[45](https://github.com/typescript-eslint/typescript-eslint/actions/runs/14020474109/job/39251785162?pr=10861#step:5:59): Argument of type '{ plugins: string[]; rules: { 'deprecation/deprecation': string; }; }' is not assignable to parameter of type 'LegacyConfig<RulesRecord, RulesRecord>'.", "  Types of property 'rules' are incompatible.", "    Type '{ 'deprecation/deprecation': string; }' is not assignable to type 'Partial<RulesRecord>'.", "      Property ''deprecation/deprecation'' is incompatible with index signature.", "        Type 'string' is not assignable to type 'RuleEntry<any[]>'."]

      23 |
      24 |       // The types should not error (e.g. https://github.com/eslint-stylistic/eslint-stylistic/issues/276)
    > 25 |       expect(lines).toHaveLength(0);
         |                     ^
      26 |     },
      27 |   );
      28 | }

      at toHaveLength (tests/flat-config-types-@types__eslint-v9.test.ts:25:21)
      at assertOutput (tools/integration-test-base.ts:197:7)
      at Object.<anonymous> (tools/integration-test-base.ts:120:9)

eslint.config.js(55,19): error TS2345: Argument of type '{ plugins: string[]; rules: { 'deprecation/deprecation': string; }; }' is not assignable to parameter of type 'LegacyConfig<RulesRecord, RulesRecord>'.
  Types of property 'rules' are incompatible.
    Type '{ 'deprecation/deprecation': string; }' is not assignable to type 'Partial<RulesRecord>'.
      Property ''deprecation/deprecation'' is incompatible with index signature.
        Type 'string' is not assignable to type 'RuleEntry<any[]>'."]

Additional Info

A few related issues & PRs:

• Bug: @types/eslint causing flat-config-types integration test failure on main #9615 -> test: add integration test against @types/eslint v9 #9622
• Repo: Integration test failures on main: vue-sfc #10879 -> chore: update vue-sfc.test.ts snapshot #10880

Related "we should automate something here" integration test tracker: #8603

💖