Don't have a full solution yet, but have some new info. Right now there are two problems:

• The Auto Smooth option is set on the object. With this on, an assert is triggered in object_to_mesh.
• A second issue introduced in rBe57ce464c21f: Fix T91600: Cycles viewport not updaing on metaball changes hid this assert at first.

This second issue is fixed with the following patch. @Brecht Van Lommel (brecht), does this look good to you?

diff --git a/intern/cycles/blender/blender_geometry.cpp b/intern/cycles/blender/blender_geometry.cpp
index fca8cb9eda3..63f865c97eb 100644
--- a/intern/cycles/blender/blender_geometry.cpp
+++ b/intern/cycles/blender/blender_geometry.cpp
@@ -80,8 +80,10 @@ Geometry *BlenderSync::sync_geometry(BL::Depsgraph &b_depsgraph,
 {
   /* Test if we can instance or if the object is modified. */
   Geometry::Type geom_type = determine_geom_type(b_ob_info, use_particle_hair);
-  BL::ID b_key_id = (BKE_object_is_modified(b_ob_info.real_object)) ? b_ob_info.real_object :
-                                                                      b_ob_info.object_data;
+  BL::ID b_key_id = (BKE_object_is_modified(b_ob_info.real_object) &&
+                     b_ob_info.is_real_object_data()) ?
+                        b_ob_info.real_object :
+                        b_ob_info.object_data;
   GeometryKey key(b_key_id.ptr.data, geom_type);
 
   /* Find shader indices. */

Still have to investigate how the first issue should be solved.

Thanks, I committed that change along with avoiding the problem with auto smooth in this file. But this code is still weak and I think there are more bugs. I need to look into it closer.

@Brecht Van Lommel (brecht) Do you have a specific bug in mind that we can put in a bug report? Currently this task is marked as a high priority bug, but it is not really actionable for me.

As far as I know, cycles shouldn't have to worry about converting curve objects to meshes anymore, since this is part of curve object evaluation-- if the curve object evaluates to a mesh, cycles will see a mesh object when it iterates through the evaluated objects.

Metaballs haven't been changed though, they will probably still need the displist -> mesh conversion.

Generally, when geometry nodes outputs a curve, it's really just a curve without any thickness. So it can't really be rendered. People use the Curve to Mesh node to give the curve a profile so that it can be rendered. This mesh will then be passed to cycles, not the original curve. Cycles does not have to do the curve-to-mesh conversion.