Crash in compareSignaturesRelated from @types/jest-in-case

[sandersn]

From the nightly DT run.

1. Clone DT, go to types/jest-in-case, run tsc:

✔ jest-in-case [master|✔]$ tsl
/home/nathansa/ts/built/local/tsc.js:88011
 throw e;
 ^

TypeError: Cannot read property 'length' of undefined
 at Object.unescapeLeadingUnderscores (/home/nathansa/ts/built/local/tsc.js:12932:19)
 at compareSignaturesRelated (/home/nathansa/ts/built/local/tsc.js:41802:103)
 at signatureRelatedTo (/home/nathansa/ts/built/local/tsc.js:43198:24)
 at signaturesRelatedTo (/home/nathansa/ts/built/local/tsc.js:43169:30)
 at structuredTypeRelatedTo (/home/nathansa/ts/built/local/tsc.js:42875:39)
 at recursiveTypeRelatedTo (/home/nathansa/ts/built/local/tsc.js:42624:64)
 at isRelatedTo (/home/nathansa/ts/built/local/tsc.js:42247:38)
 at checkTypeRelatedTo (/home/nathansa/ts/built/local/tsc.js:42029:26)
 at checkTypeRelatedToAndOptionallyElaborate (/home/nathansa/ts/built/local/tsc.js:41354:24)
 at checkApplicableSignature (/home/nathansa/ts/built/local/tsc.js:49718:26)

// @Filename: index.d.ts
declare const global: NodeJS.Global;
interface Config {
 name?: string;
 only?: boolean;
 skip?: boolean;
 [key: string]: any;
}
type Tester<Opts> = (opts: Opts, done: jest.DoneCallback) => any;
type TestCases<Opts> = ReadonlyArray<Opts> | { [name: string]: Opts };
declare function cases<Opts extends Config>(title: string, tester: Tester<Opts>, testCases: TestCases<Opts>): void;
export = cases;

// @Filename: jest-in-case-tests.ts
// (this is only the piece of the tests required to cause the crash)
import cases = require("jest-in-case");
test('array', () => {
 const tester = jest.fn(opts => {
 expect(add(opts.augend, opts.addend)).toBe(opts.total);
 });
 cases(title, tester, testCases);
});

[sandersn]

Here's a small repro:

interface Mock<Y> extends Function {
 (...args: Y): any;
}
type Tester = (opts: any, done: (...args: any[]) => any) => any;
declare function cases(tester: Tester): void;
declare function fn<Y extends any[]>(implementation?: (...args: Y) => any): Mock<Y>;
cases(fn(opts => { }));

Looks like it's probably to do with return type inference.

[sandersn]

Probably from #30114? The keywords, at least, match.