Annotate immediately-invoked functions for inlining flow control analysis · Issue #11498 · microsoft/TypeScript (original) (raw)

Per #9998 and its many offshoots, we have problems analyzing code like this

let x: string | number = "OK"; mystery(() => { x = 10; }); if (x === 10) { // Error, we believe this to be impossible
}

The problem is that we don't know if mystery invokes its argument "now" or "later" (or possibly even never!).

The converse problem appears during reads:

let x: string | number = ...; if (typeof x === 'string') { // Error, toLowerCase doesn't exist on string | number let arr = [1, 2, 3].map(n => x.toLowerCase()); }

Proposal is to allow some form of indicating that the function is invoked immediately, e.g.

declare function mystery(arg: immediate () => void): void;

This would "inline" the function body for the purposes of flow control analysis