Warn if a built-in class property is being used as a reference (it's actually a copy).

[OffsetMOSFET]

Per the discussion at godotengine/godot#80950, the array and dictionary properties of built-in class objects pass a copy instead of a reference. This behavior is different from a custom class_name, which will pass a reference.

This behavior has personally caused a lot of confusion. It took me a while to realize this was happening because I was used to the class_name behavior and knew these variable types are normally passed by reference. I had first checked the other parts of my code several times before isolating the source to this behavior.

The check I'm think of is: if the rightmost token of a gd_script line is a property of a built-in node of type array or packed*array or dictionary, and if it is not being used for assignment, the parser should warn that a property of a built-in class is pass by copy. This isn't going to catch passing it into a function that will try to modify it, but it's going to help a lot of issues quickly.

[dalexeev]

I think the warning should be triggered by the following construction:

optional_base.standard_pass_by_ref_variant_property.non_const_method()
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
              Array, Packed*Array, or Dictionary.
              Not a custom property.

Note that there may be no warning for untyped code.

[JoNax97]

While documentation is always good, I'd like to question why this inconsistency exists in the first place

[dalexeev]

See:

• Packed array properties of noncustom resources can't be resized godot#80950 (comment)
• StreamPeerBuffer.data_array.append_array() does nothing godot#106144 (comment)