From b89f2493d5d61307cabeeda5ade2bcf03cda7e2d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Rados=C5=82aw=20Cybulski?= <radoslaw.cybulski@swmansion.com>
Date: Tue, 27 Oct 2026 14:11:42 +0100
Subject: [PATCH] fix(validation): keep argument defaults inside fragments

Variable usages inside a fragment always got an undefined default value, so
VariablesInAllowedPositionRule rejected a nullable variable passed through a
fragment to a non-null argument with a default, e.g. `$intVar: Int` used as
`nonNullFieldWithDefault(arg: $intVar)` inside a fragment. GraphQL-core 3.2
accepted such queries.

Use the argument default for every variable usage, as graphql-js does since
graphql/graphql-js#4865.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 src/graphql/validation/validation_context.py  | 28 ++++++-----------
 .../test_variables_in_allowed_position.py     | 31 ++++++++++++++++++-
 2 files changed, 39 insertions(+), 20 deletions(-)

diff --git a/src/graphql/validation/validation_context.py b/src/graphql/validation/validation_context.py
index d39f94aa..175403ef 100644
--- a/src/graphql/validation/validation_context.py
+++ b/src/graphql/validation/validation_context.py
@@ -21,7 +21,6 @@
     VisitorAction,
     visit,
 )
-from ..pyutils import Undefined
 from ..utilities import TypeInfo, TypeInfoVisitor
 
 if TYPE_CHECKING:
@@ -85,33 +84,24 @@ def enter_variable_definition(self, *_args: Any) -> VisitorAction:
     def enter_variable(self, node: VariableNode, *_args: Any) -> VisitorAction:
         type_info = self._type_info
         fragment_definition = self._fragment_definition
+        fragment_variable_definition = None
         if fragment_definition:
             fragment_signature = type_info.get_fragment_signature_by_name()(
                 fragment_definition.name.value
             )
-            fragment_variable_definition = (
-                fragment_signature.variable_definitions.get(node.name.value)
-                if fragment_signature
-                else None
-            )
-            # Fragment variables have a variable default but no location default,
-            # which is what this default value represents.
-            usage = VariableUsage(
-                node,
-                type_info.get_input_type(),
-                type_info.get_parent_input_type(),
-                Undefined,
-                fragment_variable_definition,
-            )
-        else:
-            usage = VariableUsage(
+            if fragment_signature:
+                fragment_variable_definition = (
+                    fragment_signature.variable_definitions.get(node.name.value)
+                )
+        self._append_usage(
+            VariableUsage(
                 node,
                 type_info.get_input_type(),
                 type_info.get_parent_input_type(),
                 type_info.get_default_value(),
-                None,
+                fragment_variable_definition,
             )
-        self._append_usage(usage)
+        )
         return None
 
 
diff --git a/tests/validation/test_variables_in_allowed_position.py b/tests/validation/test_variables_in_allowed_position.py
index 41e37f05..56683dd6 100644
--- a/tests/validation/test_variables_in_allowed_position.py
+++ b/tests/validation/test_variables_in_allowed_position.py
@@ -374,6 +374,20 @@ def forbids_one_nullable_variable():
             )
 
     def describe_fragment_arguments_are_validated():
+        def int_to_non_null_int_when_field_argument_provides_default_value():
+            assert_valid(
+                """
+                query Query {
+                  complicatedArgs {
+                    ...A
+                  }
+                }
+                fragment A($intVar: Int) on ComplicatedArgs {
+                  nonNullFieldWithDefault(arg: $intVar)
+                }
+                """
+            )
+
         def validates_fragment_variables_defined_before_the_operation():
             assert_errors(
                 """
@@ -505,7 +519,22 @@ def int_to_non_null_int_when_optional_arg_provides_default_value():
             """
             query Query($intVar: Int) {
               complicatedArgs {
-                nonNullFieldWithDefault(nonNullIntArg: $intVar)
+                nonNullFieldWithDefault(arg: $intVar)
+              }
+            }
+            """
+        )
+
+    def int_to_non_null_int_when_argument_default_is_inside_fragment():
+        assert_valid(
+            """
+            fragment defaultArgFrag on ComplicatedArgs {
+              nonNullFieldWithDefault(arg: $intVar)
+            }
+
+            query Query($intVar: Int) {
+              complicatedArgs {
+                ...defaultArgFrag
               }
             }
             """