Fix compatibility of methods with native return type in PHP 8.1 (#1011)

(cherry picked from commit 51ad17c)

src/Error/Error.php

@@ -10,6 +10,7 @@
 use GraphQL\Language\SourceLocation;
 use GraphQL\Utils\Utils;
 use JsonSerializable;
+use ReturnTypeWillChange;
 use Throwable;
 use Traversable;
 use function array_filter;
@@ -367,6 +368,7 @@ static function (SourceLocation $loc) : array {
      * @return mixed data which can be serialized by <b>json_encode</b>,
      * which is a value of any type other than a resource.
      */
+    #[ReturnTypeWillChange]
     public function jsonSerialize()
     {
         return $this->toSerializableArray();

src/Language/SourceLocation.php

@@ -5,6 +5,7 @@
 namespace GraphQL\Language;
 
 use JsonSerializable;
+use ReturnTypeWillChange;
 
 class SourceLocation implements JsonSerializable
 {
@@ -46,6 +47,7 @@ public function toSerializableArray()
     /**
      * @return int[]
      */
+    #[ReturnTypeWillChange]
     public function jsonSerialize()
     {
         return $this->toSerializableArray();

src/Type/Definition/Type.php

@@ -11,6 +11,8 @@
 use GraphQL\Utils\Utils;
 use JsonSerializable;
 use ReflectionClass;
+use ReturnTypeWillChange;
+
 use function array_keys;
 use function array_merge;
 use function assert;
@@ -308,7 +310,8 @@ public function assertValid()
     /**
      * @return string
      */
-    public function jsonSerialize()
+    #[ReturnTypeWillChange]
+    public function jsonSerialize(): string
     {
         return $this->toString();
     }