Proxy property in setter has never type

[PabloLION]

Bug Report

🔎 Search Terms

• Proxy property in setter has never type
• Type 'any' is not assignable to type 'never'.(2322)
• Typing Proxy Constructor Proxy Handler
• Typing ProxyConstructor
• Typing ProxyHandler

🕗 Version & Regression Information

• This is a crash (misbehavior)

⏯ Playground Link

Playground link with relevant code

💻 Code

const myObj = {
  num: -1,
  arr: [1, 2, 3],
  str: '',
};

const proxy = new Proxy(myObj, {
  set(target, prop: keyof typeof myObj, value) {
    /* none of these works */
    // Ver. 1
    if (prop in target) target[prop] = value;
    // Ver. 2
    target[prop] = value;
    // Ver. 3
    target[prop as keyof typeof myObj] = value;
    // Ver. 4
    switch (prop) {
      case 'num':
      case 'str':
        target[prop] = value;
        break;
    }

    /* while these works */
    // Ver. 5
    switch (prop) {
      case 'num':
        target[prop] = value;
        break;
      case 'str':
        target[prop] = value;
        break;
    }
    // Ver. 6
    switch (prop) {
      case 'num':
        target[prop] = value;
        break;
      default:
        target[prop] = value;
        break;
    }

    return true;
  },
});

console.log('proxy : ', proxy);

🙁 Actual behavior

With TypeScript 4.5.4 (current latest) and currently Nightly version, this code is throwing error Type 'any' is not assignable to type 'never'.(2322) for the version 1,2,3,4 yet for version 5 and 6 no error is reported. If we remove one of the num prop or str, all the errors are gone.

Is there an explanation for this design?

Also asked on stack overflow

🙂 Expected behavior

We can use all of version 1,2,3,4 without having this TS2322 error.

[fatcerberus]

Is there an explanation for this design?

Yes. See #30769. In most of your setups, the type of prop is not narrowed down from "num" | "str" | "arr"; since you're writing to target[prop] rather than reading from it, that produces an intersection (not a union) of all the possible types it could refer to. If there's no overlap, the intersection collapses to never, because there's no possible value that works for all of them.

[beepeep]

Bug Report

🔎 Search Terms

• Proxy property in setter has never type
• Type 'any' is not assignable to type 'never'.(2322)
• Typing Proxy Constructor Proxy Handler
• Typing ProxyConstructor
• Typing ProxyHandler

🕗 Version & Regression Information

• This is a crash (misbehavior)

⏯ Playground Link

Playground link with relevant code

💻 Code

const myObj = {
  num: -1,
  arr: [1, 2, 3],
  str: '',
};

const proxy = new Proxy(myObj, {
  set(target, prop: keyof typeof myObj, value) {
    /* none of these works */
    // Ver. 1
    if (prop in target) target[prop] = value;
    // Ver. 2
    target[prop] = value;
    // Ver. 3
    target[prop as keyof typeof myObj] = value;
    // Ver. 4
    switch (prop) {
      case 'num':
      case 'str':
        target[prop] = value;
        break;
    }

    /* while these works */
    // Ver. 5
    switch (prop) {
      case 'num':
        target[prop] = value;
        break;
      case 'str':
        target[prop] = value;
        break;
    }
    // Ver. 6
    switch (prop) {
      case 'num':
        target[prop] = value;
        break;
      default:
        target[prop] = value;
        break;
    }

    return true;
  },
});

console.log('proxy : ', proxy);

🙁 Actual behavior

With TypeScript 4.5.4 (current latest) and currently Nightly version, this code is throwing error Type 'any' is not assignable to type 'never'.(2322) for the version 1,2,3,4 yet for version 5 and 6 no error is reported. If we remove one of the num prop or str, all the errors are gone.

Is there an explanation for this design?

Also asked on stack overflow

🙂 Expected behavior

We can use all of version 1,2,3,4 without having this TS2322 error.

[PabloLION]

Thank you for explaining me all these, @fatcerberus.
I did see "This conditional type returns never when it should return the true branch" (#31751) in the "not a bug" part, but it seems irrelevant.
And you are right, now I understand it's an intersection. (it can be "string" and can be "number" so it must be intersection of "string" and "number", meanly, "never"). However,,

There are two more doubts I didn't call out:

1. What is the correct pattern to achieve this purpose, except "as any"? how can I fix it? Please offer some links.
2. after removing the property "str" or "num", the type of target[prop] changed to number | number[] or string|number[]. Shouldn't (one of)these be "never" as well?

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.