diff --git a/src/Turbo/TurboExtensionEnabler.php b/src/Turbo/TurboExtensionEnabler.php
index e77f9fdcba9..621eac414e5 100644
--- a/src/Turbo/TurboExtensionEnabler.php
+++ b/src/Turbo/TurboExtensionEnabler.php
@@ -33,7 +33,7 @@
 final class TurboExtensionEnabler
 {
 
-	public const EXPECTED_EXTENSION_VERSION = '6888887';
+	public const EXPECTED_EXTENSION_VERSION = '6f9d639';
 
 	private static bool $active = false;
 
diff --git a/src/Type/Generic/TemplateTypeHelper.php b/src/Type/Generic/TemplateTypeHelper.php
index 90c983455cd..c3662134b41 100644
--- a/src/Type/Generic/TemplateTypeHelper.php
+++ b/src/Type/Generic/TemplateTypeHelper.php
@@ -52,6 +52,17 @@ public static function resolveTemplateTypes(
 					return $traverse($type);
 				}
 
+				// the template type standing in for itself, e.g. in the class that declares it,
+				// keeps the bound narrowed at this occurrence, like `int|string` of a `K of int|string|object`
+				// in the key of `array<K, V>`
+				if (
+					$newType instanceof TemplateType
+					&& $newType->getName() === $type->getName()
+					&& $newType->getScope()->equals($type->getScope())
+				) {
+					return $traverse($newType->isArgument() ? $type->toArgument() : $type);
+				}
+
 				if ($newType instanceof ErrorType && !$keepErrorTypes) {
 					return $traverse($type->getDefault() ?? $type->getBound());
 				}
diff --git a/tests/PHPStan/Analyser/nsrt/bug-15321.php b/tests/PHPStan/Analyser/nsrt/bug-15321.php
new file mode 100644
index 00000000000..f7fc3026709
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-15321.php
@@ -0,0 +1,127 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug15321;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @template K of string|int|object
+ * @template V
+ */
+interface Map
+{
+	/** @return (K is array-key ? array<K, V> : array<array-key, V>) */
+	public function toArray(): array;
+
+	/** @return array<K, V> */
+	public function toArrayPlain(): array;
+
+	/** @return non-empty-array<K, V> */
+	public function toNonEmptyArray(): array;
+
+	/** @param array<K, V> $array */
+	public function merge(array $array): void;
+}
+
+/** @param Map<string, int> $map */
+function test(Map $map): void
+{
+	assertType('array<string, int>', $map->toArray());
+	assertType('array<string, int>', $map->toArrayPlain());
+	assertType('non-empty-array<string, int>', $map->toNonEmptyArray());
+}
+
+/**
+ * @template K of string|int|object
+ * @template V
+ */
+final class ArrayMap
+{
+
+	/** @var array<K, V> */
+	private array $items;
+
+	/**
+	 * @param array<K, V> $promoted
+	 * @param array<K, V> $items
+	 */
+	public function __construct(private array $promoted, array $items)
+	{
+		$this->items = $items;
+	}
+
+	/** @param array<K, V> $array */
+	public function merge(array $array): void
+	{
+		assertType('array<K of int|string (class Bug15321\ArrayMap, argument), V (class Bug15321\ArrayMap, argument)>', $array);
+		assertType('array<K of int|string (class Bug15321\ArrayMap, argument), V (class Bug15321\ArrayMap, argument)>', $this->items);
+		assertType('array<K of int|string (class Bug15321\ArrayMap, argument), V (class Bug15321\ArrayMap, argument)>', $this->promoted);
+	}
+
+	/** @return array<K, V> */
+	public function toArray(): array
+	{
+		return $this->items;
+	}
+
+}
+
+/** @param ArrayMap<int, string> $map */
+function testArrayMap(ArrayMap $map): void
+{
+	assertType('array<int, string>', $map->toArray());
+}
+
+/**
+ * @template K of string|int|object
+ * @template V
+ */
+trait MapTrait
+{
+
+	/** @return array<K, V> */
+	public function traitToArray(): array
+	{
+		return [];
+	}
+
+}
+
+/**
+ * @template K of string|int|object
+ * @template V
+ * @extends Map<K, V>
+ */
+interface SortedMap extends Map
+{
+
+	/** @return array<K, V> */
+	public function toSortedArray(): array;
+
+}
+
+/**
+ * @template K of string|int|object
+ * @template V
+ */
+abstract class MapWithTrait
+{
+
+	/** @use MapTrait<K, V> */
+	use MapTrait;
+
+}
+
+/**
+ * @param SortedMap<string, int> $sortedMap
+ * @param MapWithTrait<string, int> $mapWithTrait
+ */
+function testInherited(SortedMap $sortedMap, MapWithTrait $mapWithTrait): void
+{
+	assertType('array<string, int>', $sortedMap->toArray());
+	assertType('array<string, int>', $sortedMap->toArrayPlain());
+	assertType('array<string, int>', $sortedMap->toSortedArray());
+	assertType('array<string, int>', $mapWithTrait->traitToArray());
+}
diff --git a/turbo-ext/src/TemplateTypeHelper.cpp b/turbo-ext/src/TemplateTypeHelper.cpp
index ecc2a561e53..7dc93b002d3 100644
--- a/turbo-ext/src/TemplateTypeHelper.cpp
+++ b/turbo-ext/src/TemplateTypeHelper.cpp
@@ -260,6 +260,39 @@ void TemplateTypeHelper::resolveTemplateTypesCallback(zval *state0, zval *state1
 				return;
 			}
 
+			/* the template type standing in for itself keeps the bound
+			 * narrowed at this occurrence - see the twin */
+			bool newIsTemplate;
+			if (UNEXPECTED(!pt_tth_is_template(newType.raw(), newIsTemplate))) return;
+			if (newIsTemplate) {
+				zv::Val newName = pt_tth_name(newType.raw());
+				if (UNEXPECTED(newName.isUndef())) return;
+				if (zend_string_equals(Z_STR_P(newName.raw()), Z_STR_P(name.raw()))) {
+					zv::Val newScope = pt_type_call(Z_OBJ_P(newType.raw()), PT_LC("getscope"), 0, NULL);
+					if (UNEXPECTED(newScope.isUndef())) return;
+					zv::Val scope = pt_type_call(Z_OBJ_P(type), PT_LC("getscope"), 0, NULL);
+					if (UNEXPECTED(scope.isUndef())) return;
+					if (UNEXPECTED(!zv::Ref(newScope.raw()).isObject())) {
+						zend_type_error("phpstan_turbo: getScope() must return an object");
+						return;
+					}
+					zv::Val scopesEqual = pt_type_call(Z_OBJ_P(newScope.raw()), PT_LC("equals"), 1, scope.raw());
+					if (UNEXPECTED(scopesEqual.isUndef())) return;
+					if (zend_is_true(scopesEqual.raw())) {
+						zv::Val newIsArgument = pt_type_call(Z_OBJ_P(newType.raw()), PT_LC("isargument"), 0, NULL);
+						if (UNEXPECTED(newIsArgument.isUndef())) return;
+						if (!zend_is_true(newIsArgument.raw())) {
+							(void) pt_type_traverser_traverse(return_value, traverse, type);
+							return;
+						}
+						zv::Val argument = pt_type_call(Z_OBJ_P(type), PT_LC("toargument"), 0, NULL);
+						if (UNEXPECTED(argument.isUndef())) return;
+						(void) pt_type_traverser_traverse(return_value, traverse, argument.raw());
+						return;
+					}
+				}
+			}
+
 			bool isError;
 			if (UNEXPECTED(!pt_type_instanceof_ce(newType.raw(), pt_ce_error_type, isError))) return;
 			if (isError && !zend_is_true(keepErrorTypes)) {