microsoft / microsoft/TypeScript

Missing constraint error in instantiation expression for a nested class

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Bug Domain: check: Variance Relationships Help Wanted
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描述

### 🔎 Search Terms

instantiation expression caching class expression declaration type arguments type parameter

### 🕗 Version & Regression Information

- This changed in PR: https://github.com/microsoft/TypeScript/pull/59931 , cc @weswigham

### ⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250701#code/MYGwhgzhAECCA8AVaBeaA7ArgWwEYFMAnAPmgG8AoaaANzBE3wEIAuaRAbiumAAsBLEABNW0AJLoIAFzDpg+RAE8ADvnhSV+APYAzOADoAQvASJi5jtAD0V6FIEwIvLZmHQihLYQrdpYKfzA0IaoPOBQwUjuAB5S+OhCMLChsKSU1NTKYITxUqKc3AC+XIUUQA

### 💻 Code

```ts
class A {
value!: T;
child!: InstanceType>>; // this should error

static B = class B {
parent!: T;
};
}
```

### 🙁 Actual behavior

It doesn't error

### 🙂 Expected behavior

it should error with:
```
Type 'A' does not satisfy the constraint 'A'.
Type 'T' is not assignable to type 'number'.(2344)
```

like it did in 5.6: [TS playground](https://www.typescriptlang.org/play/?ts=5.6.3#code/MYGwhgzhAECCA8AVaBeaA7ArgWwEYFMAnAPmgG8AoaaANzBE3wEIAuaRAbiumAAsBLEABNW0AJLoIAFzDpg+RAE8ADvnhSV+APYAzOADoAQvASJi5jtAD0V6FIEwIvLZmHQihLYQrdpYKfzA0IaoPOBQwUjuAB5S+OhCMLChsKSU1NTKYITxUqKc3AC+XIUUQA)

### Additional information about the issue

The problem here is that the situation is a little bit circular and variances worker gets hits in reentrant manner. The nested call returns `emptyArray` that is a signal for `structuredTypeRelatedToWorker` to return `Ternary.Unknown`.

So `checkTypeArguments` called by `getInstantiatedSignatures` doesn't report an error. In the old version of the code, this would be re-called after the variance worker would completely exit and the error would be raised.

I see 2 ways to solve this:
- ignore the introduced caching based `inVarianceComputation` to avoid spoiling this
- defer `checkTypeArguments` in `getInstantiatedSignatures` (well, call it with `/*reportErrors*/ false` and defer anoher call with `/*reportErrors*/ true`)

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调研方向

从链接的 TypeScript Playground 复现开始,跟踪 getInstantiatedSignatures 经由 checkTypeArguments、structuredTypeRelatedToWorker 以及 inVarianceComputation 路径的执行过程。将其行为与 5.6 Playground 链接进行比较,并确定延迟检查或未缓存的检查应如何报告约束错误。当当前复现产生诊断 2344,且不破坏相关的类型检查行为时,即视为完成。

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评估

技术栈
typescript
领域
compilers
Issue 类型
缺陷
难度
4/5
预计耗时
3-5 天
活跃度
停滞
描述清晰度
基本清楚
新手友好度
35/100

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