Fix accepting BenevolentUnionType as part of GenericObjectType

src/Type/Generic/TemplateTypeVariance.php

@@ -102,7 +102,7 @@ public function isValidVariance(Type $a, Type $b): TrinaryLogic
 			return TrinaryLogic::createYes();
 		}
 
-		if ($b instanceof BenevolentUnionType) {
+		if ($b instanceof BenevolentUnionType && !$a instanceof BenevolentUnionType) {
 			$results = [];
 			foreach ($b->getTypes() as $innerType) {
 				$results[] = $this->isValidVariance($a, $innerType);

tests/PHPStan/Rules/Methods/ReturnTypeRuleTest.php

@@ -449,4 +449,9 @@ public function testTemplateStringBound(): void
 		$this->analyse([__DIR__ . '/data/template-string-bound.php'], []);
 	}
 
+	public function testBug4605(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-4605.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Methods/data/bug-4605.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace Bug4605;
+
+/**
+ * @phpstan-template TKey
+ * @psalm-template TKey of array-key
+ * @psalm-template T
+ * @template-extends IteratorAggregate<TKey, T>
+ * @template-extends ArrayAccess<TKey|null, T>
+ */
+interface Collection extends \Countable, \IteratorAggregate, \ArrayAccess {}
+
+class Boo {
+	/**
+	 * @param Collection<array-key, string> $collection
+	 * @return Collection<array-key, string>
+	 */
+	public function foo(Collection $collection): Collection
+	{
+		return $collection;
+	}
+
+	/**
+	 * @param Collection<int, string> $collection
+	 * @return Collection<int, string>
+	 */
+	public function boo(Collection $collection): Collection
+	{
+		return $collection;
+	}
+}