Godot: Typed Arrays and Dictionaries without initial value are shared

[qarmin]

Godot version:
3.1 8dd00ed

OS/device including version:
Windows 10

Issue description:
When I append value to array, other array have this value inside.
This is broken code

extends Node2D

var offerts_parameters_array : Array
var offerts_array : Array

func _ready() -> void:
    offerts_parameters_array.resize(0)
    offerts_array.resize(0)

    offerts_parameters_array.append(["OPA"])
    offerts_parameters_array.append(["OPA"])
    offerts_parameters_array.append(["OPA"])
    offerts_parameters_array.append("SOPA")
    offerts_array.append(["OA"])
    offerts_array.append(["OA"])
    offerts_array.append("SOA")
    print(offerts_parameters_array)
    print(offerts_array)

and this is Output

[[OPA], [OPA], [OPA], SOPA, [OA], [OA], SOA]
[[OPA], [OPA], [OPA], SOPA, [OA], [OA], SOA]

When I didn't use typed GDScript then it seems to work correct:

extends Node2D

var offerts_parameters_array = []
var offerts_array = []

func _ready() -> void:
    offerts_parameters_array.resize(0)
    offerts_array.resize(0)

    offerts_parameters_array.append(["OPA"])
    offerts_parameters_array.append(["OPA"])
    offerts_parameters_array.append(["OPA"])
    offerts_parameters_array.append("SOPA")
    offerts_array.append(["OA"])
    offerts_array.append(["OA"])
    offerts_array.append("SOA")
    print(offerts_parameters_array)
    print(offerts_array)

and this is Output

[[OPA], [OPA], [OPA], SOPA]
[[OA], [OA], SOA]

[bojidar-bg]

Flagging as bug since we should either enforce giving an initial value or make sure it is initialized to new, unshared arrays.

[remorse107]

Looks like decorating with export (Array) also causes the break to occur even when the variable isn't Typed.

[akien-mga]

Confirmed in 6e600706.

[TheCire]

Hmm, Im wondering if the default value of

var array: Array should actually be null and cause a crash if not initialized. it would help to enforce giving a default value and would avoid the bug all together.

[MidZik]

That might require more changes, since you aren't allowed to assign null to variables statically typed as built-in types that don't extend Object (like Array or Dictionary).

[LeonardMeagher2]

I think typed variables should require an initial value, that's really what "var array: Array should actually be null and cause a crash if not initialized." is trying to get you to infer, but I think it's better to be explicit and tell someone "This isn't going to work as expected without an initial value".

[MidZik]

Yeah, I misunderstood.

Like you said, better to have an explicit warning.

[akien-mga]

As seen in #27997, it seems to be the same for Dictionary.