Multimeshinstance3D use_custom_data can't be set by gdscript

[croxis]

Tmp2.zip

Godot version

4.0 alpha 2

System information

Linux

Issue description

use_custom_data and cuse_colors can't be set via gdscript when setting up a multimesh in a multimeshinstance3d. Or I failed to read something correctly in the documentation.

init
ERROR: Condition "instance_count > 0" is true.
   at: set_use_custom_data (scene/resources/multimesh.cpp:289)
ERROR: Condition "instance_count > 0" is true.
   at: set_use_colors (scene/resources/multimesh.cpp:280)
init: 1 false

Steps to reproduce

In an otherwise empty project:

extends MultiMeshInstance3D

func _ready():
	print("init")
	multimesh.instance_count = 1
	multimesh.use_custom_data = true
	multimesh.use_colors = true
	print("init: ", multimesh.instance_count, " ", multimesh.use_custom_data)

Minimal reproduction project

No response

[vfig]

This is a documentation issue. Both Optimization Using MultiMeshes and Animating thousands of fish with MultiMeshInstance note that the transform format, and color/custom options must be set before setting instance_count. To quote the latter:

instance_count specifies how many instances of the mesh you want to draw. For now, leave instance_count at 0 because you cannot change any of the other parameters while instance_count is larger than 0. We will set instance_count in GDScript later.

But this restriction is not mentioned at all in the MultiMesh api reference entry.