The <partial_name>_iteration local is missing when rendering heterogeneous collections

[meagar]

Steps to reproduce

• In a controller, create a heterogeneous array of objects responding to to_partial_path, such as a set of ActiveRecord models in an STI hierarchy, and assign it to @collection
• Render the collection using render @collection or render partial: @collection
• Attempt to use <template_name>_iteration in the rendered partial, and find that it isn't defined

See https://github.com/meagar/partial-iteration-test for a simple POC, specifically the commit that introduces the bug and screenshots of the broken/expected behavior.

Expected behavior

A variable named <partial_name>_iteration should be set to an instance of ActionView::PartialIteration

Actual behavior

The variable is unset

System configuration

Rails version:

Rails 5.0.1

Ruby version:

ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-darwin15]