Statically incorrect type annotations are permitted on for loop variables

[PatchMixolydic]

Godot version

v4.2.dev5.official [e3e2528], v4.2.dev.custom_build [7b49cbade]

System information

Godot v4.2.dev5 - Debian GNU/Linux 12 (bookworm) 12 - X11 - Vulkan (Mobile) - dedicated NVIDIA GeForce GTX 1060 3GB (nvidia) - AMD Ryzen 7 1700X Eight-Core Processor (16 Threads)

Issue description

Type annotations that are obviously incorrect are accepted on for loop variables, leading to runtime errors (specifically, "Trying to assign a value of type 'int' to a variable of type 'String'"). The implementation PR, #80247, notes that cases where the runtime type is different will cause runtime errors. However, this issue occurs even when the types should be statically known.

Steps to reproduce

1. Create a project, populate it with a Node3D, and save the scene.
2. Attach the following script to the Node3D:

extends Node3D

func _ready() -> void:
	for x: String in [1, 2, 3]:
		print(x)

3. Note that the script passes typecheck.
4. Run the current project, setting the current scene as the default scene if prompted.
5. Note that a runtime error occurs.

Extra info

Compare the script above to the following script:

extends Node3D

func _ready() -> void:
	var x: Array[String] = [1, 2, 3]

This correctly produces an error at compile time:

Line 4: Cannot include a value of type "int" as "String".
Line 4: Cannot have an element of type "int" in an array of type "Array[String]".

Minimal reproduction project

godot issue.zip

[dalexeev]

for x: String in [1, 2, 3]:

I'm not sure if this is a bug. By default, array literals are untyped (Array aka Array[Variant]), we make literals typed only in some contexts. The following code correctly generates an error:

var a: Array[int] = [1, 2, 3]
# Compile time error: Unable to iterate on value of type
# "Array[int]" with variable of type "String".
for x: String in a:
    print(x)

var x: Array[String] = [1, 2, 3]

This is one of the cases where we are implicitly converting a literal to a typed array. Maybe we should make the literal typed in this case too, but I'm not sure if that's a good idea.