Fix oneOf introspection (#3528)

RELEASE.md

@@ -0,0 +1,4 @@
+release type: patch
+
+This release fixes an introspection issue when requesting `isOneOf` on built-in
+scalars, like `String`.

TWEET.md

@@ -0,0 +1,6 @@
+🆕 Release $version is out! Thanks to $contributor for the PR 👏
+
+This release fixes an introspection issue when requesting `isOneOf` on built-in
+scalars, like `String`.
+
+Get it here 👉 $release_url

strawberry/schema/schema.py

@@ -380,6 +380,9 @@ def _warn_for_federation_directives(self) -> None:
 
     def _extend_introspection(self) -> None:
         def _resolve_is_one_of(obj: Any, info: Any) -> bool:
+            if "strawberry-definition" not in obj.extensions:
+                return False
+
             return obj.extensions["strawberry-definition"].is_one_of
 
         instrospection_type = self._schema.type_map["__Type"]

tests/schema/test_one_of.py

@@ -259,3 +259,20 @@ def test_introspection():
     assert not result.errors
 
     assert result.data == {"__type": {"name": "ExampleInputTagged", "isOneOf": True}}
+
+
+def test_introspection_builtin():
+    query = """
+        query {
+          __type(name: "String") {
+            name
+            isOneOf
+          }
+        }
+    """
+
+    result = schema.execute_sync(query)
+
+    assert not result.errors
+
+    assert result.data == {"__type": {"name": "String", "isOneOf": False}}