--noImplicitAny codefixes infer anonymous object types despite appropriate primitives

[JoshuaKGoldberg]

TypeScript Version: 3.3.0-dev.20190108

Search Terms: noimplicitany infer code fix codefix quick suggested object literal primitive

Code

function checkEnd(text) {
    text.length;
    text.indexOf("z");
    text.charAt(0);
}

Expected behavior:

--noImplicitAny suggested fix on the text parameter:

checkEnd(text: string) {

Actual behavior:

function checkEnds(text: { length: any; indexOf: (arg0: string) => void; charAt: (arg0: number) => void; }) {

Related Issues: #13243 (parent tracking quick fixes); #28991 for similar issues with interfaces/types

At what point can TypeScript be confident something is a string, number, array, etc.? In the above case of a parameter whose usages match up to exactly one built-in primitive (string here), can it be inferred to that primitive instead of a very long anonymous object type that happens to be a subset of the primitive's?

Proposal: If a --noImplicitAny fix would add in an anonymous object type that is a subset of exactly one built-in primitive type, such as number, string, or boolean, it should add that primitive type instead.