Ensure 'in' does not operate on primitive types

Bug Report

🔎 Search Terms

operator in exception crash unhandled

🕗 Version & Regression Information

This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

const mustBeObject = (thing: object) => thing

const hasKey = <A extends object | null | string | number, K extends string | number | symbol>(
    thing: A,
    key: K,
): boolean => {
    if (thing && typeof thing === 'object') {
        mustBeObject(thing);
        return key in thing; // no error
    }
    return key in thing; // error
};

hasKey(123, 'hello'); 

🙁 Actual behavior

TS should expect A to be an object

🙂 Expected behavior

TS didn't detect the potential crash

---

This is a follow-up to #41317. In #41928, we decided that we should implement a more conservative check that only ensures that the resolved constraint of the right operand to the in operator is not assignable to a primitive. This is a negative check ensuring that the type of the right operand does not explicitly extend a primitive type. This solution does not yet cover the original example shown above.

The alternative we discussed was a positive check that the type of the right expression cannot possibly extend a primitive type. We could do this either by checking that the type of right operand (not its resolved constraint!) is not assignable to a primitive or by checking that this type is assignable to object.

I am reopening this issue because with #43183 (🎉) the original reason why we went with the more conservative (from a breaking change point of view) check will soon not be a limitation any more. Therefore, code like

   if (typeof val === 'object' && '__isMaybe' in val) {

does not represent a problem anymore because val can be narrowed even if its type is a type parameter as long as it extends a union (see https://github.com/microsoft/TypeScript/pull/41928#issuecomment-743356094 for the original discussion).

Start with the linked TypeScript Playground and the in operator example, then read the discussion of #41317, #41928, and #43183. Compare the current handling of generic right operands against the stated primitive-type crash; done means the checker reliably rejects unsafe in expressions without breaking the narrowing behavior described in the issue.