GDScript: Implement pattern guards for match statement

[vnen]

Within a match statement, it is now possible to add guards in each branch:

var a = 0
match a:
	0 when false: print("does not run")
	0 when true: print("but this does")

This allows more complex logic for deciding which branch to take.

Closes godotengine/godot-proposals#4775

EDIT: Changed the -> if separator to the new keyword when, as discussed.

[dalexeev]

The code looks good to me! 👍 There is a little hack with the pattern guard is SuiteNode, not ExpressionNode. In theory, we can get unrelated warnings or something like that, but I did not find any related bugs when I tested it.

The tests are good, but maybe we should add some more? For example a test that an assignment is not allowed inside a pattern guard.

I'm still not sure about the syntax, so I suggest discussing this at the next GDScript team meeting.

[dalexeev]

I noticed that the pattern guard is executed even if none of the patterns matched.

func f() -> bool:
    print("f")
    return true

func _ready() -> void:
    var x := 1
    match x:
        2 -> if f():
            print("A")
        _:
            print("B")

Prints:

f
B

[vnen]

I noticed that the pattern guard is executed even if none of the patterns matched.

func f() -> bool:
    print("f")
    return true

func _ready() -> void:
    var x := 1
    match x:
        2 -> if f():
            print("A")
        _:
            print("B")

Prints:

f
B

Fixed now. Added a test for this case as well as the one for assignment.

[adamscott]

Discussed during a GDScript Team Meeting: this PR will be modified my @vnen to use the when keyword to introduce the if statement. This PR would be ready for 4.2 afterwards.

[vnen]

I updated the PR to use then new when keyword.

[dalexeev]

Looks good to me and works great, it won't even break compatibility if someone uses when as an identifier. Thank you George!

var when = 1
match when:
    when when when: # I like it.
        when

[YuriSizov]

Thanks!