Only reuse memoized default values for the type they were coerced to

src/graphql/type/definition.py

@@ -1254,16 +1254,16 @@ class GraphQLDefaultInput:  # noqa: PLW1641
     literal: ConstValueNode | None
     """GraphQL literal default value, or None if a runtime value is provided instead."""
 
-    __slots__ = "_memoized_coerced_value", "literal", "value"
+    __slots__ = "_memoized_coercion", "literal", "value"
 
     def __init__(
         self, value: Any = Undefined, literal: ConstValueNode | None = None
     ) -> None:
         self.value = value
         self.literal = literal
-        # Used to memoize the result of coercing the default value (see
-        # coerce_default_value() in the utilities).
-        self._memoized_coerced_value: Any = Undefined
+        # Used to memoize the result of coercing the default value, together with
+        # the type it was coerced to (see coerce_default_value() in the utilities).
+        self._memoized_coercion: tuple[GraphQLInputType | None, Any] = None, Undefined
 
     def __eq__(self, other: object) -> bool:
         return self is other or (

src/graphql/utilities/coerce_input_value.py

@@ -355,10 +355,12 @@ def coerce_default_value(
     # field on the GraphQLDefaultInput object. (Contrary to GraphQL.js, which
     # memoizes on the input value itself, this also works for the immutable
     # variable signatures that reuse this function for fragment arguments.)
+    # It is only reused for the same type, as a default can be used with
+    # different types, e.g. after extend_schema() replaced the type.
     default_input = input_value.default
     if default_input is not None:
-        coerced_value = default_input._memoized_coerced_value  # noqa: SLF001
-        if coerced_value is Undefined:
+        coerced_type, coerced_value = default_input._memoized_coercion  # noqa: SLF001
+        if coerced_type is not input_value.type:
             coerced_value = (
                 coerce_input_literal(default_input.literal, input_value.type)
                 if default_input.literal is not None
@@ -375,7 +377,7 @@ def coerce_default_value(
                     f" to be valid, found: {found}."
                 )
                 raise TypeError(msg)
-            default_input._memoized_coerced_value = coerced_value  # noqa: SLF001
+            default_input._memoized_coercion = input_value.type, coerced_value  # noqa: SLF001
         return coerced_value
 
     # The deprecated internal default value is used as is.

tests/utilities/test_coerce_input_value.py

@@ -716,3 +716,14 @@ def coerce_input_value(value):
         # Call a second time
         assert coerce_default_value(input_value) == "hello"
         assert coerce_input_value_calls == ["hello"]
+
+    def memoizes_coercion_per_type():
+        color_enum = GraphQLEnumType("Color", {"RED": 0, "BLUE": 1})
+        shared_default = GraphQLDefaultInput("RED")
+
+        color_value = GraphQLInputField(color_enum, default=shared_default)
+        string_value = GraphQLInputField(GraphQLString, default=shared_default)
+
+        assert coerce_default_value(color_value) == 0
+        assert coerce_default_value(string_value) == "RED"
+        assert coerce_default_value(color_value) == 0

tests/utilities/test_extend_schema.py

@@ -4,7 +4,7 @@
 
 import pytest
 
-from graphql import graphql_sync
+from graphql import ExecutionResult, graphql_sync
 from graphql.error import GraphQLSyntaxError
 from graphql.language import parse, print_ast
 from graphql.type import (
@@ -383,6 +383,35 @@ def extends_inputs_by_adding_new_fields():
             ),
         )
 
+    def coerces_default_values_with_extended_input_types():
+        schema = build_schema(
+            """
+            type Query {
+              someInput(arg: SomeInput = {}): String
+            }
+
+            input SomeInput {
+              oldField: String
+            }
+            """
+        )
+        extend_ast = parse(
+            """
+            extend input SomeInput {
+              newField: String = "new"
+            }
+            """
+        )
+        extended_schema = extend_schema(schema, extend_ast)
+        root_value = {"someInput": lambda _info, arg: str(arg)}
+
+        def query(schema: GraphQLSchema) -> ExecutionResult:
+            return graphql_sync(schema, "{ someInput }", root_value)
+
+        assert query(schema) == ({"someInput": "{}"}, None)
+        assert query(extended_schema) == ({"someInput": "{'newField': 'new'}"}, None)
+        assert query(schema) == ({"someInput": "{}"}, None)
+
     def extends_scalars_by_adding_new_directives():
         schema = build_schema(
             """