Checker loses Array subclass types for mixed index and property key unions #3626

class Child extends Array<string> {}
function read(child: Child, key: '0' | 'length') {
    const wrong: boolean = child[key];
}

SharpTS at 72455db66 accepts this with --no-tsconfig --strict --noEmit. TypeScript 6.0.3 reports TS2322 with --ignoreConfig --strict --target ES2022 --noEmit. The canonical key '0' reads a string element and 'length' reads a number, so the combined result should have type string | number.

Found while verifying #3623, which covers unions where every key is a canonical index.