Trying to modify const Dictionary results in confusing error

[KoBeWi]

Godot version:

fb2465b

Issue description:

const a = {}

func _ready() -> void:
	a["test"] = 1

This results in Cannot get index "test" from "{}". (parse error)

If I'm not supposed to modify const Dictionary, the error should clearly say so.

[ThakeeNathees]

there was already an issue for this: #44241

[KoBeWi]

It's not the same issue. In my case it works when I do var a = {}

[YuriSizov]

This should be clear after #71051.