Cannot assign Array[A_Extend] to Array[A] in GDScript

[luckyabsoluter]

Tested versions

v4.3.stable.official [77dcf97]

System information

v4.3.stable.official [77dcf97]

Issue description

When attempting to assign an Array of a subclass (Array[A_Extend]) to an Array of its parent class (Array[A]), the GDScript compiler throws the following error:

Value of type "Array[A_Extend]" cannot be assigned to a variable of type "Array[A]".

This behavior contradicts the expected polymorphism in object-oriented programming where a subclass should be assignable to a parent class reference.

Expected Behavior:

The array_A_Extend (an array of subclass A_Extend) should be assignable to array_A (an array of parent class A) without any errors.

Actual Behavior:

An error is thrown, preventing the assignment:
Value of type "Array[A_Extend]" cannot be assigned to a variable of type "Array[A]".

Steps to reproduce

Assign

1. Create a new Godot project.
2. Add the following script to a Node:

extends Node

class A:
    pass

class A_Extend extends A:
    pass

func _ready():
    var array_A: Array[A] = []
    var array_A_Extend: Array[A_Extend] = []

    array_A = array_A_Extend  # Error: Value of type "Array[A_Extend]" cannot be assigned to a variable of type "Array[A]".
    #array_A.assign(array_A_Extend)  # bug workaround

3. Check the script for errors.

Argument

1. Create a new Godot project.
2. Add the following script to a Node:

extends Node

class A:
    pass

class A_Extend extends A:
    pass

func _ready():
    var array_A: Array[A] = []
    var array_A_Extend: Array[A_Extend] = []

    need_A(array_A_Extend)  # Error: Invalid argument for "need_A()" function: argument 1 should be "Array[A]" but is "Array[A_Extend]".
    #array_A.assign(array_A_Extend)  # bug workaround
    #need_A(array_A)

func need_A(p: Array[A]):
    pass

3. Check the script for errors.

Argument but No error

1. Create a new Godot project.
2. Add the following script to a Node:

extends Node

class A:
    func _to_string():
        return "A"

class A_Extend extends A:
    func _to_string():
        return "A_Extend"

func _ready():
    need_A([A_Extend.new()])

func need_A(array_A: Array[A]):
    print(array_A)

3. No error!

Minimal reproduction project (MRP)

N/A

[tetrapod00]

Maybe duplicate of #95113

[dalexeev]

Duplicate of:

• Functions accept typed arrays and cannot inherit conversions #80586
• Typed array function arguments don't accept typed arrays of subclass #83876
• TypedArray does not work with Inheritence #89959
• static type arrya cannot understand extends(subclass) #93063
• Polymorphism doesn't work on a typed arrays #93084

See:

• Wikipedia - Covariance and contravariance (computer science)
• GDScript reference - Typed arrays
• Behavior of typed arrays godot-proposals#7364

This is not a bug. Arrays are passed by reference; they do not copy the entire content. Otherwise you would get:

var a: Array[Node2D]
var b: Array[Node]
b = a # Now `b` refers to the same array as `a`.
b.append(Label.new()) # Valid, `Label` is a subtype of `Node`.
print(a[0]) # Invalid, `Label` is **not** a subtype of `Node2D`.

Probably the only way this would work is if the GDScript supported read/write qualifiers. But it's too difficult.

Thanks for the contribution nonetheless!