Regression - able to use union containing non-indexing type as index

[RyanCavanaugh]

TypeScript Version: master at bd40583

Search Terms: array index union error

Code

declare const arr1: (string | string[])[];
declare const arr2: number[];
const j = arr2[arr1[0]];

Expected behavior: Error - Type string[] cannot be used as an index type

Actual behavior: No error

Playground Link: Link

Related Issues: Regressed during fix of #26409 (edit: #26281 ?)

[RyanCavanaugh]

Ref #26712 & #26698

[ahejlsberg]

Looks to me like this is caused by #26281.

[weswigham]

Righto, so this is a bit interesting. Since the index distributes to number[string] | number[string[]] and noImplicitAny is off, number[string] silently becomes any and then we don't bother to check number[string[]]. I can fix it so we still do the validity check even if we won't use the type; just thought it was interesting~