bug(HMR + @boundary): template error boundaries disappear with HMR refresh

从链接的 StackBlitz 复现项目中的 src/main.ts 开始，比较 @boundary 示例的初始渲染与模板或样式经过 HMR 刷新后的行为。使用 RiskyComponent 中硬编码的 Error 重现故障；当 boundary 的 fallback 保持可见，同时 boundary 外部的内容不变时，即表示完成。

Which @angular/* package(s) are the source of the bug?

core, Don't known / other

Is this a regression?

Yes

Description

The new error boundaries (@boundary) coming to v22.2.0 in developer preview are neat, but I believe I found a bug with template boundaries and HMR.

In the documentation page, the first example block is the following (but for this example I added a <p> tag outside the boundary for reference):

@boundary {
  <app-risky-component />
} @error {
  <p>Something went wrong!</p>
}
<p>Outside of boundary</p>

I have hardcoded the risky component to always throw an Error, so you only see the @error fallback content.

export class RiskyComponent {
  constructor() {
    throw new Error('forced error');
  }
}

If you are serving the app with HMR and you make an HMR change like to the template or style declaration:

• The boundary block content disappears
• BUT the content outside of the boundary block, like <p>Outside of the boundary</p>, is just fine.

Please provide a link to a minimal reproduction of the bug

https://stackblitz.com/edit/stackblitz-starters-td3er4br?file=src%2Fmain.ts

Please provide the exception or error you saw

`@boundary` template block disappears upon HMR refresh.

Please provide the environment you discovered this bug in (run ng version)

Angular CLI       : 22.2.0-next.6
Angular           : 22.2.0-next.7
Node.js           : 22.22.3
Package Manager   : npm 10.8.2
Operating System  : linux x64

┌───────────────────────────┬───────────────────┬───────────────────┐
│ Package                   │ Installed Version │ Requested Version │
├───────────────────────────┼───────────────────┼───────────────────┤
│ @angular/build            │ 22.2.0-next.6     │ ^22.2.0-next.6    │
│ @angular/cli              │ 22.2.0-next.6     │ ^22.2.0-next.6    │
│ @angular/common           │ 22.2.0-next.7     │ ^22.2.0-next.7    │
│ @angular/compiler         │ 22.2.0-next.7     │ ^22.2.0-next.7    │
│ @angular/compiler-cli     │ 22.2.0-next.7     │ ^22.2.0-next.7    │
│ @angular/core             │ 22.2.0-next.7     │ ^22.2.0-next.7    │
│ @angular/forms            │ 22.2.0-next.7     │ ^22.2.0-next.7    │
│ @angular/platform-browser │ 22.2.0-next.7     │ ^22.2.0-next.7    │
│ @angular/router           │ 22.2.0-next.7     │ ^22.2.0-next.7    │
│ rxjs                      │ 7.8.2             │ ^7.8.1            │
│ typescript                │ 6.0.3             │ ^6.0.3            │
└───────────────────────────┴───────────────────┴───────────────────┘

Anything else?

I tend to overthink edge cases going into issues, but I am trying to not do so currently lol. I have some edge cases in mind I may try out and bring up. If I do, I will edit those into this block with notes, or just cross them out.

My todo to check:

• Reset (will always fail with hardcoded error)
• Reset (random chance to actually not throw error)
• Dynamic views, and their variants with the types of reset cases above