GDScript: Implement pattern guards for match statement

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.

2 files changed, 81 insertions, 0 deletions

diff --git a/modules/gdscript/tests/scripts/runtime/features/match_with_pattern_guards.gd b/modules/gdscript/tests/scripts/runtime/features/match_with_pattern_guards.gd
new file mode 100644
index 0000000000..4cb51f8512
--- /dev/null
+++ b/modules/gdscript/tests/scripts/runtime/features/match_with_pattern_guards.gd
@@ -0,0 +1,71 @@
+var global := 0
+
+func test():
+	var a = 0
+	var b = 1
+
+	match a:
+		0 when b == 0:
+			print("does not run" if true else "")
+		0 when b == 1:
+			print("guards work")
+		_:
+			print("does not run")
+
+	match a:
+		var a_bind when b == 0:
+			prints("a is", a_bind, "and b is 0")
+		var a_bind when b == 1:
+			prints("a is", a_bind, "and b is 1")
+		_:
+			print("does not run")
+
+	match a:
+		var a_bind when a_bind < 0:
+			print("a is less than zero")
+		var a_bind when a_bind == 0:
+			print("a is equal to zero")
+		_:
+			print("a is more than zero")
+
+	match [1, 2, 3]:
+		[1, 2, var element] when element == 0:
+			print("does not run")
+		[1, 2, var element] when element == 3:
+			print("3rd element is 3")
+
+	match a:
+		_ when b == 0:
+			print("does not run")
+		_ when b == 1:
+			print("works with wildcard too.")
+		_:
+			print("does not run")
+
+	match a:
+		0, 1 when b == 0:
+			print("does not run")
+		0, 1 when b == 1:
+			print("guard with multiple patterns")
+		_:
+			print("does not run")
+
+	match a:
+		0 when b == 0:
+			print("does not run")
+		0:
+			print("regular pattern after guard mismatch")
+
+	match a:
+		1 when side_effect():
+			print("should not run the side effect call")
+		0 when side_effect():
+			print("will run the side effect call, but not this")
+		_:
+			assert(global == 1)
+			print("side effect only ran once")
+
+func side_effect():
+	print("side effect")
+	global += 1
+	return false
diff --git a/modules/gdscript/tests/scripts/runtime/features/match_with_pattern_guards.out b/modules/gdscript/tests/scripts/runtime/features/match_with_pattern_guards.out
new file mode 100644
index 0000000000..452d1ff4bf
--- /dev/null
+++ b/modules/gdscript/tests/scripts/runtime/features/match_with_pattern_guards.out
@@ -0,0 +1,10 @@
+GDTEST_OK
+guards work
+a is 0 and b is 1
+a is equal to zero
+3rd element is 3
+works with wildcard too.
+guard with multiple patterns
+regular pattern after guard mismatch
+side effect
+side effect only ran once