Fix QueryPlan when querying __typename on a union type (#1295)

Author: spawnia

CHANGELOG.md

@@ -9,6 +9,12 @@ You can find and compare releases at the [GitHub release page](https://github.co
 
 ## Unreleased
 
+## v15.0.2
+
+### Fixed
+
+- Fix `QueryPlan` when querying `__typename` on a `union` type
+
 ## v15.0.1
 
 ### Fixed

src/Type/Definition/QueryPlan.php

@@ -152,14 +152,14 @@ private function analyzeSelectionSet(SelectionSetNode $selectionSet, Type $paren
         $implementors = [];
         foreach ($selectionSet->selections as $selectionNode) {
             if ($selectionNode instanceof FieldNode) {
-                assert($parentType instanceof HasFieldsType, 'ensured by query validation');
-
                 $fieldName = $selectionNode->name->value;
 
                 if ($fieldName === Introspection::TYPE_NAME_FIELD_NAME) {
                     continue;
                 }
 
+                assert($parentType instanceof HasFieldsType, 'ensured by query validation and the check above which excludes union types');
+
                 $type = $parentType->getField($fieldName);
                 $selectionType = $type->getType();