Annotate immediately-invoked functions for inlining flow control analysis

Per #9998 and its many offshoots, we have problems analyzing code like this ``` ts 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: ```ts 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. ``` ts declare function mystery(arg: immediate () => void): void; ``` This would "inline" the function body for the purposes of flow control analysis

Posted by RyanCavanaugh on Oct 10, 2016, 7:19 PM

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