Before Phoenix 1.7 error_tag was a function defined for you in the ErrorHelpers module (source).

Since 1.7 there is a CoreComponents module. If has an <.input /> component that renders an input field with error feedback using <.error />. So you could use that:

<.input field={@form[:some_field]} type="hidden" />

This will generate a hidden input field and the error feedback.

If you’d rather display the error yourself you can just access it directly: @form[:some_field].errors and render it as you like.

The Phoenix Form Bindings guide discusses using phx-feedback attribute for styling and hiding error messages.

Updated: Just noticed LiveView 1.0 will drop support for phx-feedback.

Not sure how this wouldn’t seem relevant, but if you follow what happens as part of <.input> it would show that it does the following eventually:

<.error :for={msg <- Enum.map(@form[:gender].errors, &translate_error(&1)}><%= msg %></.error>