Repository: angular
Issue: NG8011: Node matches the X slot of the Y component, but will not be projected into the specific slot because the surrounding @if has more than one node at its root.
Labels: area: core,area: compiler,core: control flow

### Which @angular/* package(s) are the source of the bug?

compiler

### Is this a regression?

Yes

### Description

I have the following code which causes a warning during compilation, but the warning suggests fixes that don't apply to the code in question.

```html
  <mat-form-field subscriptSizing="dynamic">
    <mat-label>Phone Number</mat-label>
    <input
      matInput
      [formControl]="form.controls.phoneNumber"
      required
      type="tel"
    />
    @if (form.controls.phoneNumber.hasError('isPhoneNumber')) {
      <mat-error>Must be a valid U.S. phone number</mat-error>
    }
  </mat-form-field>
```

This caused the following compiler warning, followed by some suggestions to fix it.

> NG8011: Node matches the "mat-error, [matError]" slot of the "MatFormField" component, but will not be projected into the specific slot because the surrounding @if has more than one node at its root.

The bug here is that **there is not more than one node at the root of the @if**. Suggestion 1 is basically already in-place, suggestion 2 is impossible, and section 3 is already in-place.

Note that this is not a duplicate of #53477, because #53477 is a case where the content does need to be split/wrapped.

### Please provide a link to a minimal reproduction of the bug

_No response_

### Please provide the exception or error you saw

```true
The compiler error is as follows:


Warning: src/app/@shared/send-it/send-it.component.html:14:7 - warning NG8011: Node matches the "mat-error, [matError]" slot of the "MatFormField" component, but will not be projected into the specific slot because the surrounding @if has more than one node at its root. To project the node in the right slot, you can:

1. Wrap the content of the @if block in an <ng-container/> that matches the "mat-error, [matError]" selector.
2. Split the content of the @if block across multiple @if blocks such that each one only has a single projectable node at its root.
3. Remove all content from the @if block, except for the node being projected.
Note: the host component has `preserveWhitespaces: true` which may cause whitespace to affect content projection.

This check can be disabled using the `extendedDiagnostics.checks.controlFlowPreventingContentProjection = "suppress" compiler option.`

14       <mat-error>Must be a valid U.S. phone number</mat-error>
         ~~~~~~~~~~~

  src/app/@shared/send-it/send-it.component.ts:15:16
    15   templateUrl: './send-it.component.html',
                      ~~~~~~~~~~~~~~~~~~~~~~~~~~
    Error occurs in the template of component SendItComponent.
```


### Please provide the environment you discovered this bug in (run `ng version`)

```true
Angular CLI: 17.1.1
Node: 18.18.0
Package Manager: yarn 1.22.19
OS: darwin arm64

Angular: 17.1.0
... animations, cdk, common, compiler, compiler-cli, core, forms
... language-service, localize, material, platform-browser
... platform-browser-dynamic, router, service-worker

Package                         Version
---------------------------------------------------------
@angular-devkit/architect       0.1701.1
@angular-devkit/build-angular   17.1.1
@angular-devkit/core            17.1.1
@angular-devkit/schematics      17.1.1
@angular/cli                    17.1.1
@angular/pwa                    17.1.1
@schematics/angular             17.1.1
rxjs                            7.8.1
typescript                      5.3.3
zone.js                         0.14.3
```


### Anything else?

This happens in many other similar (one root node inside the `@if`) instances--not always MatFormField, even--across my application.