[prefer-readonly-parameter-types] Cannot read property 'flags' of undefined

[AriPerkkio]

Hello, prefer-readonly-parameter-types rule seems to crash in certain cases. This issue was spotted by automated CI run - it is not blocking my development or anything.

• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have read the FAQ and my problem is not listed.

Repro

// .eslintrc.js
module.exports = {
  root: true,
  env: {
    es6: true,
    node: true,
  },
  parser: "@typescript-eslint/parser",
  parserOptions: {
    ecmaVersion: 2020,
    sourceType: "module",
    ecmaFeatures: {
      jsx: true,
    },
    tsconfigRootDir: __dirname,
    project: ["./tsconfig.json"],
  },
  plugins: ["@typescript-eslint"],
  rules: {
    "@typescript-eslint/prefer-readonly-parameter-types": "error",
  },
};

// tsconfig.json
{
  "compilerOptions": {
    "target": "es6",
    "allowJs": false,
    "skipLibCheck": true,
    "esModuleInterop": true,
    "allowSyntheticDefaultImports": true,
    "strict": true,
    "noImplicitReturns": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "forceConsistentCasingInFileNames": true,
    "lib": ["esnext", "dom"],
    "module": "commonjs",
    "moduleResolution": "node",
    "outDir": "dist"
  },
  "include": ["./index.ts"]
}

This is as minimal as I could narrow the issue down.

type MyType<T> = {
  [K in keyof T]: "cat" | "dog" | T[K];
};

function method<A extends any[] = string[]>(value: MyType<A>) {
  return value;
}

method(["cat", "dog"]);
method<"mouse"[]>(["cat", "mouse"]);

Expected Result

ESLint should not crash.

Actual Result

TypeError: Cannot read property 'flags' of undefined
Occurred while linting /workspaces/eslint-remote-tester/node_modules/.cache-eslint-remote-tester/querycap/webappkit/@querycap/reactutils/must.tsx:7
    at Object.isUnionType (/workspaces/eslint-remote-tester/node_modules/tsutils/typeguard/2.8/type.js:69:18)
    at unionTypeParts (/workspaces/eslint-remote-tester/node_modules/tsutils/util/type.js:92:19)
    at isPropertyReadonlyInType (/workspaces/eslint-remote-tester/node_modules/tsutils/util/type.js:151:21)
    at isReadonlyPropertyFromMappedType (/workspaces/eslint-remote-tester/node_modules/tsutils/util/type.js:197:12)
    at /workspaces/eslint-remote-tester/node_modules/tsutils/util/type.js:179:21
    at someTypePart (/workspaces/eslint-remote-tester/node_modules/tsutils/util/type.js:100:52)
    at isReadonlyPropertyIntersection (/workspaces/eslint-remote-tester/node_modules/tsutils/util/type.js:172:12)
    at Object.isPropertyReadonlyInType (/workspaces/eslint-remote-tester/node_modules/tsutils/util/type.js:161:43)
    at isTypeReadonlyObject (/workspaces/eslint-remote-tester/node_modules/@typescript-eslint/eslint-plugin/dist/util/isTypeReadonly.js:72:28)
    at isTypeReadonlyRecurser (/workspaces/eslint-remote-tester/node_modules/@typescript-eslint/eslint-plugin/dist/util/isTypeReadonly.js:127:30)

Additional Info

Crash reports from real projects

Rule: prefer-readonly-parameter-types

• Message: Cannot read property 'flags' of undefined Occurred while linting /workspaces/eslint-remote-tester/node_modules/.cache-eslint-remote-tester/querycap/webappkit/@querycap/reactutils/must.tsx:7
• Path: querycap/webappkit/@querycap/reactutils/must.tsx
• Link

   5 |
   6 | export function must<TPrepare extends Readonly<Array<any>>>(usePrepare: () => TPrepare) {
>  7 |   return function <TProps>(render: (props: TProps, ...prepare: NonNullableArray<TPrepare>) => JSX.Element | null) {
   8 |     const C = ({ "data-prepare": dataPrepare, ...props }: TProps & { ["data-prepare"]: NonNullableArray<TPrepare> }) =>
   9 |       render(props as any, ...(dataPrepare as any));
  10 |

TypeError: Cannot read property 'flags' of undefined
Occurred while linting /workspaces/eslint-remote-tester/node_modules/.cache-eslint-remote-tester/querycap/webappkit/@querycap/reactutils/must.tsx:7
    at Object.isUnionType (/workspaces/eslint-remote-tester/node_modules/tsutils/typeguard/2.8/type.js:69:18)
    at unionTypeParts (/workspaces/eslint-remote-tester/node_modules/tsutils/util/type.js:92:19)
    at isPropertyReadonlyInType (/workspaces/eslint-remote-tester/node_modules/tsutils/util/type.js:151:21)
    at isReadonlyPropertyFromMappedType (/workspaces/eslint-remote-tester/node_modules/tsutils/util/type.js:197:12)
    at /workspaces/eslint-remote-tester/node_modules/tsutils/util/type.js:179:21
    at someTypePart (/workspaces/eslint-remote-tester/node_modules/tsutils/util/type.js:100:52)
    at isReadonlyPropertyIntersection (/workspaces/eslint-remote-tester/node_modules/tsutils/util/type.js:172:12)
    at Object.isPropertyReadonlyInType (/workspaces/eslint-remote-tester/node_modules/tsutils/util/type.js:161:43)
    at isTypeReadonlyObject (/workspaces/eslint-remote-tester/node_modules/@typescript-eslint/eslint-plugin/dist/util/isTypeReadonly.js:72:28)
    at isTypeReadonlyRecurser (/workspaces/eslint-remote-tester/node_modules/@typescript-eslint/eslint-plugin/dist/util/isTypeReadonly.js:127:30)

Versions

package	version
@typescript-eslint/eslint-plugin	4.23.0
@typescript-eslint/parser	4.23.0
TypeScript	4.2.4
ESLint	7.26.0
node	v14.15.1

[JoshuaKGoldberg]

@kirjs and I had a splendid time pairing on this: turns out tsutils crashes on a mapped type when the type mapping doesn't have modifiers.

ajafff/tsutils#136

[rafaelss95]

I'm not entirely sure if it's a duplication, but it looks a lot like the one reported in #1742 (error logs are pretty similar).

[AriPerkkio]

@rafaelss95 the stack trace from tsutils seems identical so this is very likely a duplicate. I'm usually very careful not to file duplicate issues - no idea how I missed that issue here.
As this issue already has a fix proposal linked I guess it makes sense to keep this one open.

[JoshuaKGoldberg]

As of #6428, the v6 branch uses ts-api-tools. Which has the same bug as tsutils. Fix is in review here: JoshuaKGoldberg/ts-api-utils#18