fix: Derived element json serialization (tefra#1035)

tests/formats/dataclass/serializers/test_dict.py

@@ -3,6 +3,7 @@
 from tests.fixtures.books import BookForm, Books
 from tests.fixtures.datatypes import Telephone
 from tests.fixtures.wrapper import Wrapper
+from xsdata.formats.dataclass.models.generics import AnyElement, DerivedElement
 from xsdata.formats.dataclass.serializers import DictEncoder, DictFactory
 from xsdata.models.datatype import XmlDate
 from xsdata.models.xsd import Attribute
@@ -107,3 +108,23 @@ def test_next_value(self):
         expected = expected[:-1]
         actual = [name for name, value in self.encoder.next_value(book)]
         self.assertEqual(expected, actual)
+
+    def test_generics(self):
+        self.obj = AnyElement(
+            children=[
+                AnyElement(qname="foo", text="bar"),
+                DerivedElement(qname="bar", value="1"),
+                DerivedElement(qname="bar", value=2),
+            ]
+        )
+        result = self.encoder.encode(self.obj)
+        expected = {
+            "attributes": {},
+            "children": [
+                {"attributes": {}, "children": [], "qname": "foo", "text": "bar"},
+                {"qname": "bar", "value": "1"},
+                {"qname": "bar", "value": 2},
+            ],
+        }
+
+        self.assertEqual(expected, result)

xsdata/formats/dataclass/models/generics.py

@@ -33,7 +33,7 @@ class AnyElement:
 class DerivedElement(Generic[T]):
     """Generic model wrapper for type substituted elements.
 
-    Example: eg. <b xsi:type="a">...</b>
+    Example: e.g. <b xsi:type="a">...</b>
 
     Args:
         qname: The element's qualified name

xsdata/formats/dataclass/typing.py

@@ -7,6 +7,7 @@
     Optional,
     Tuple,
     Type,
+    TypeVar,
     Union,
 )
 
@@ -152,6 +153,11 @@ def evaluate_attributes(annotation: Any, **_: Any) -> Result:
 
 def evaluate_element(annotation: Any, tokens: bool = False) -> Result:
     """Validate annotations for a xml element."""
+
+    # Only the derived element value field is allowed a typevar
+    if isinstance(annotation, TypeVar) and annotation.__bound__ is object:
+        annotation = object
+
     types = (annotation,)
     origin = get_origin(annotation)
     args = get_args(annotation)