[L]ooking a little closely at the ECMAScript spec, I do think that there is a mismatch between what we have and what gets created in the runtime. RegExpBuiltinExec says that the following are created unconditionally:

• an index property
• an input property
• a property at index 0
• a groups property that is always set, but may be undefined

So RegExpExecArray should look something kind of like

// src/lib/es5.d.ts

interface RegExpExecArray extends Array<string> {
    // always present
    index: number;

    // always present
    input: string;

    // always present - helpful for '--noUncheckedIndexedAccess'
    0: string;
}

// src/lib/es2018.regexp.d.ts

interface RegExpExecArray extends Array<string> {
    // always present - helpful for '--exactOptionalPropertyTypes'
    groups: { [key: string]: string } | undefined;
}

Derived from content posted by @DanielRosenwasser in #50211 (comment)

In additionl to the above, it seems like RegExpMatchArray/RegExpExecArray are sometimes both returned from functions that undergo the same internal code paths. So we should probably ensure that one is compatible with the other, and have a test that ensures as much.