`Compiler bug: Unresolved return` on doubled functions with while loops with await within

[Scraprogrammer]

Versions

(list all versions where you have replicated the bug)

• Godot: 4.7.0
• GUT: 9.7.0
• OS: Windows 11

The Bug

Functions of a double that have a while loop within them with an await will cause an error to be produced. The error will not stop the execution of the test and asserts will pass, but the test will still fail because an uncaught error was thrown. The error:

ERROR: Compiler bug: Unresolved return.
   at: write_return (modules/gdscript/gdscript_byte_codegen.cpp:1846)
   GDScript backtrace (most recent call first):
       [0] create_script_from_source (res://addons/gut/dynamic_gdscript.gd:28)
       [1] create_script_from_source (res://addons/gut/utils.gd:368)
       [2] _create_script_no_warnings (res://addons/gut/doubler.gd:185)
       [3] _create_double (res://addons/gut/doubler.gd:215)
       [4] _partial_double (res://addons/gut/doubler.gd:308)
       [5] partial_double (res://addons/gut/doubler.gd:321)
       [6] _smart_double (res://addons/gut/test.gd:372)
       [7] partial_double (res://addons/gut/test.gd:2662)
       [8] test_name_does_not_matter (res://tests/unit/test_dbl_fail.gd:6)
       [9] _run_test (res://addons/gut/gut.gd:622)
       [10] _test_the_scripts (res://addons/gut/gut.gd:835)
       [11] test_scripts (res://addons/gut/gut.gd:1045)
       [12] run_tests (res://addons/gut/gui/GutRunner.gd:198)
       [13] _run_tests (res://addons/gut/cli/gut_cli.gd:239)
       [14] main (res://addons/gut/cli/gut_cli.gd:297)
       [15] _init (res://addons/gut/gut_cmdln.gd:39)

Steps To Reproduce

Have a class to be doubled:

class_name FailTestClass
extends Node

var should_stop_waiting: bool = false


func wait_with_while() -> bool:
	while not should_stop_waiting:
		await get_tree().create_timer(1.0).timeout
		should_stop_waiting = true
	return true

Have a test:

extends GutTest


func test_name_does_not_matter() -> void:
	# Arrange
	var dbl: FailTestClass = partial_double(FailTestClass).new()

	# Assert
	assert_not_null(dbl)

Run the text and get the error listed above.

Extra context

• Same behavior if a double is used instead of the partial double.
• If await get_tree().create_timer(1.0).timeout is removed from the function - the error disappears.

[Scraprogrammer]

And looks like another bot reply...

[bitwes]

I was able to replicate the issue. It appears to happen with any await in a function that has a declared return type.

Causes error:

func bool_await() -> bool:
	await get_tree().process_frame
	return true

Does not cause error:

func bool_await():
	await get_tree().process_frame
	return true

GUT does not include return types in the source code generated for a Double. I was able to confirm that if GUT does include the return type in the Double's method signature the error does not happen. The fix is to add return types to method signatures in Doubles.

** Edit
Removed old workaround and examples that no longer apply as of GUT 9.7.1

[Scraprogrammer]

Thank you! :)
Atm, I think I'll be able to get away with removing the return types and adding some comments/TODOs for the future me, since I don't have a lot of places that got affected by the error.

[bitwes]

I merged a temporary fix for this into the godot_4_7 branch. The error will no longer cause tests to fail.

Fixing the issue properly will require adding return type clauses to doubled methods but that opened a giant can of method metadata worms that I've yet to tame.

[bitwes]

Status

For anyone making your way here from the output of your tests, here's what's happening.

As of GUT 9.7.1, the error is being ignored and will no longer cause tests to fail. The error still appears in the log. Godot's source indicates that this message shouldn't be reachable, but everyone knows that if you comment "should not get here", someone will eventually end up there.

Workaround

If you remove the return type from the method generating the error, then the error will go away. It's not a great workaround, but its the only one we got.

Full Solution

Maybe Godot will remove the error. If not, a complete fix will require adding return type clauses to methods when generating the source of a Double/Partial Double. GUT uses metadata about methods to create doubles, it does not parse the script being doubled. The information in the metadata is currently not sufficient to determine the right return type in all cases.

This comment will change as we progress towards a fix.