Duplicate @key values don't throw clear exception in if/else case

[SteveSandersonMS]

As reported here: #18307 (comment)

With this code,

@if (cat.IsBlack)
{
    <input @key="@cat.Id" type="checkbox"
        @onchange=@(eventArgs => { cat.IsBlack= (bool)eventArgs.Value; HandleCatColor(cat); })
        checked />
}
else
{
    <input @key="@cat.Id" type="checkbox"
    @onchange=@(eventArgs => { cat.IsBlack= (bool)eventArgs.Value; HandleCatColor(cat); }) />
}

This should throw a clear exception about reuse of the same @key value with a different sequence ID when toggling cat.IsBlack as per docs:

Ensure that values used for @key don't clash. If clashing values are detected within the same parent element, Blazor throws an exception

However because of the if/else, it doesn't throw but instead exhibits broken behavior.

This isn't super high priority because it's not a valid code pattern anyway. The issue is just that the error detection doesn't catch this particular case.

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.