Objects with "Optimize for Instancing" option and different material appear with one material

[usakhelo]

Discovered this bug while working on ActiveShade PR. To reproduce do the following:

1. Create an object
2. Apply appleseed modifier and check "Optimize for Instancing"
3. Make one or more instances of the object
4. Assign different materials to the instances
5. Render. Objects are rendered with one material due to assembly_map cache. First object assembly contains object and it's material and subsequent instances use the same assembly.

[usakhelo]

I'm wondering if it's a bug or it should be this way?
It's single "object assembly" with its own set of object instances and materials that gets duplicated with assembly instances, right? So they will have the same material probably.

[dictoon]

This is a long standing limitation of appleseed: instances of an assembly cannot override materials used in that assembly, that means that all instances of an assembly use the same materials.