microsoft / microsoft/TypeScript
Incorrect type resolution with nested constructors
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Description
### 🔎 Search Terms
nested constructors type resolution anti-pattern
### 🕗 Version & Regression Information
- This changed between versions ______ and _______
- This changed in commit or PR _______
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
- I was unable to test this on prior versions because I only report bugs to be helpful, but I don't have time to dive deeper myself.
### ⏯ Playground Link
https://www.typescriptlang.org/play/?#code/KYDwDg9gTgLgBAE2AYwDYEMrDsiA7AZ3gCF0DgAuOAbwCgBIPYAdwAoBKKu++gawEs8CKgHIAgiIaMWHLnAFDRxEXAC+DdetyF4YuAF44TZjgwECcUDGBCLpcjTW1tROMQNGWpshas2EFnrUTsQAdAoIQA
### 💻 Code
```ts
export declare const Base: {
new(): {
kind: 'A'
new(): { kind: 'B' }
}
}
const A = new class extends Base { }
const B = new class extends A { }
B.kind // shows 'A', but should be 'B'
```
### 🙁 Actual behavior
If a ctor returns another ctor, it repeats the first level. In the above example, B.kind is 'A', but since we extended and instantiated twice, it should be 'B'.
### 🙂 Expected behavior
After extending twice, it should reach the inner class, instead of repeating the outer one.
### Additional information about the issue
I can only reproduce this bug with this weird experimental anti-pattern - so on its own probably not a priority at all, but types should probably reflect the runtime state as much as possible, even in weird edge cases such as this, and it might indicate a deeper underlying issue.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the linked TypeScript Playground reproduction and trace type resolution for the two nested `new class extends ...` expressions. The work is done when the example reports `B.kind` as `'B'` instead of `'A'` after the two extensions and instantiations.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Quiet
- Clarity
- Mostly clear
- Newbie friendliness
- 48/100