ValidationSummary tag helper should not render the tag if there are not items to render

[sebastienros]

With <div asp-validation-summary="ModelOnly"></div>:

On GET

On POST with errors

On POST without errors in the model

In this last screenshot, there are no model errors, however there are non-model errors. The div should not be rendered.

[dougbu]

@sebastienros is this a dupe of #2372?

Or are you requesting a behaviour change for the "ModelOnly requested case" from checking IsValid to checking whether ModelState includes errors for the current Model?

I ask in part because "model errors" versus "non-model errors" doesn't align with the meaning of ModelOnly. To make that more clear, consider a template view for a POCO class somewhere in the model tree. <div asp-validation-summary="ModelOnly"</div> in that view will list errors in the ModelState[ViewData.TemplateInfo.HtmlFieldPrefix] key but nothing else. On the other hand, <div asp-validation-summary="All"></div> will list every error in ModelState i.e. for properties of the POCO class, the current instance of that class, everything above it, and so on.

[dougbu]

Talked offline w/ @sebastienros and the #2372 may be useful for his scenarios. But the change in behaviour I described above fixes the particular bug he hit here.

[dougbu]

eee1a9f