Indexing tuple intersection type beyond length produces unexpected type

Bug Report

🔎 Search Terms

tuple intersection index type

⏯ Playground Link

Playground Link

💻 Code

type T = [number, boolean] & { x: string };
declare const t: T;
const a = t[0]; // number
const b = t[1]; // boolean
const c = t[2]; // number | boolean !?

🙁 Actual behavior

c is typed as number | boolean

🙂 Expected behavior

c should be typed as undefined

I'm not sure if this is a bug, or intended behavior. But it was surprising. If the type is just a tuple, indexing beyond its length produced undefined. But if it is part of an intersection, the element type is changed to number | boolean, as if it is no longer a tuple, but an array. However, indexing 0 or 1 still produces the correct type for those element positions.

Start with the linked TypeScript Playground and reproduce the tuple-intersection indexed access shown in the issue. Trace the compiler's type-checking path for out-of-bounds tuple indexing when an intersection is present, then add coverage showing that t[2] is typed as undefined while t[0] and t[1] retain their element types.