PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/contributing-docs/commit-message-guidelines.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.dev application / infrastructure changes
• Other... Please describe:

What is the current behavior?

NG8011 fires when a @let declaration sits next to the node at the root of a control flow block:

<form-field>
  <input />
  @if (hasError()) {
    @let message = 'Email is required';
    <span error>{{ message }}</span>
  }
</form-field>

The warning says <span error> won't be projected into the [error] slot, but it is. Since #58607 the compiler skips @let declarations when it looks for the single root node of a block. The diagnostic does its own count in TcbControlFlowContentProjectionOp.shouldCheck and still treats the @let as a second root node.

Issue Number: Fixes #71172

What is the new behavior?

shouldCheck skips @let declarations, so the diagnostic agrees with what the compiler does. I added two tests: one for the case above (no diagnostic), and one with a @let plus a stray text node, which is still reported. The first one fails without the change.

Does this PR introduce a breaking change?

• Yes
• No

Other information

It only removes a false-positive warning; the generated code doesn't change.