[Fix] ES2025+: GeneratorResumeAbrupt: properly handle return completions

Author: ljharb

2025/GeneratorResumeAbrupt.js

@@ -22,30 +22,34 @@ module.exports = function GeneratorResumeAbrupt(generator, abruptCompletion, gen
 	var state = GeneratorValidate(generator, generatorBrand); // step 1
 
 	if (state === 'SUSPENDED-START') { // step 2
-		SLOT.set(generator, '[[GeneratorState]]', 'COMPLETED'); // step 3.a
-		SLOT.set(generator, '[[GeneratorContext]]', null); // step 3.b
-		state = 'COMPLETED'; // step 3.c
+		SLOT.set(generator, '[[GeneratorState]]', 'COMPLETED'); // step 2.a
+		SLOT.set(generator, '[[GeneratorContext]]', null); // step 2.b
+		state = 'COMPLETED'; // step 2.c
 	}
 
 	var value = abruptCompletion.value();
 
 	if (state === 'COMPLETED') { // step 3
-		return CreateIteratorResultObject(value, true); // steps 3.a-b
+		if (abruptCompletion.type() === 'return') { // step 3.a
+			return CreateIteratorResultObject(value, true); // step 3.a.i
+		}
+		return abruptCompletion['?'](); // step 3.b
 	}
 
 	if (state !== 'SUSPENDED-YIELD') {
 		throw new $TypeError('Assertion failed: generator state is unexpected: ' + state); // step 4
 	}
-	if (abruptCompletion.type() === 'return') {
-		// due to representing `GeneratorContext` as a function, we can't safely re-invoke it, so we can't support sending it a return completion
-		return CreateIteratorResultObject(SLOT.get(generator, '[[CloseIfAbrupt]]')(NormalCompletion(abruptCompletion.value())), true);
-	}
 
 	var genContext = SLOT.get(generator, '[[GeneratorContext]]'); // step 5
 
 	SLOT.set(generator, '[[GeneratorState]]', 'EXECUTING'); // step 8
 
-	var result = genContext(value); // steps 6-7, 8-11
+	if (abruptCompletion.type() === 'return') {
+		// due to representing `GeneratorContext` as a function, we can't safely re-invoke it, so we can't support sending it a return completion
+		return CreateIteratorResultObject(SLOT.get(generator, '[[CloseIfAbrupt]]')(NormalCompletion(value)), true);
+	}
+
+	var result = genContext(value); // steps 6-7, 9-11
 
 	return result; // step 12
 };

test/methods/GeneratorResumeAbrupt.js

@@ -86,15 +86,50 @@ module.exports = function (t, year, GeneratorResumeAbrupt, extras) {
 		'generator with return completion -> done iter result'
 	);
 
+	// test re-entrant return() throws TypeError
+	var reentrantGenerator = {};
+	SLOT.set(reentrantGenerator, '[[GeneratorState]]', 'SUSPENDED-YIELD');
+	SLOT.set(reentrantGenerator, '[[GeneratorBrand]]', brand);
+	SLOT.set(reentrantGenerator, '[[GeneratorContext]]', null);
+
+	var enterCount = 0;
+	var reentrantCloseIfAbrupt = function () {
+		enterCount += 1;
+		if (enterCount > 1) {
+			// guard against infinite recursion in case the fix is not applied
+			throw new RangeError('re-entrant call was not prevented');
+		}
+		// re-entrant call while generator is executing
+		GeneratorResumeAbrupt(reentrantGenerator, ReturnCompletion(undefined), brand);
+		return { sentinel: true };
+	};
+	SLOT.set(reentrantGenerator, '[[CloseIfAbrupt]]', reentrantCloseIfAbrupt);
+
+	t.equal(enterCount, 0, 'closeIfAbrupt not called yet');
+
+	t['throws'](
+		function () { GeneratorResumeAbrupt(reentrantGenerator, ReturnCompletion(42), brand); },
+		TypeError,
+		'throws TypeError when return() is called re-entrantly during [[CloseIfAbrupt]]'
+	);
+
+	t.equal(enterCount, 1, 'closeIfAbrupt was entered exactly once before throwing');
+
 	SLOT.set(generator, '[[GeneratorState]]', 'SUSPENDED-START');
+	t['throws'](
+		function () { GeneratorResumeAbrupt(generator, completion, brand); },
+		42,
+		'SUSPENDED-START with throw completion transitions to COMPLETED then throws'
+	);
+	t.equal(SLOT.get(generator, '[[GeneratorState]]'), 'COMPLETED', 'state is completed');
+	t.equal(SLOT.get(generator, '[[GeneratorContext]]'), null, 'context is unset');
+
 	t.deepEqual(
-		GeneratorResumeAbrupt(generator, completion, brand),
+		GeneratorResumeAbrupt(generator, ReturnCompletion(42), brand),
 		{
 			value: 42,
 			done: true
 		},
-		'completed state -> done iter result'
+		'COMPLETED state with return completion -> done iter result'
 	);
-	t.equal(SLOT.get(generator, '[[GeneratorState]]'), 'COMPLETED', 'state is completed');
-	t.equal(SLOT.get(generator, '[[GeneratorContext]]'), null, 'context is unset');
 };