fix: update typevar handling when default is not set

pydantic/_internal/_generate_schema.py

@@ -1449,15 +1449,14 @@ def _unsubstituted_typevar_schema(self, typevar: typing.TypeVar) -> core_schema.
 
         bound = typevar.__bound__
         constraints = typevar.__constraints__
-        not_set = object()
-        default = getattr(typevar, '__default__', not_set)
+        default = getattr(typevar, '__default__', None)
 
-        if (bound is not None) + (len(constraints) != 0) + (default is not not_set) > 1:
+        if (bound is not None) + (len(constraints) != 0) + (default is not None) > 1:
             raise NotImplementedError(
                 'Pydantic does not support mixing more than one of TypeVar bounds, constraints and defaults'
             )
 
-        if default is not not_set:
+        if default is not None:
             return self.generate_schema(default)
         elif constraints:
             return self._union_schema(typing.Union[constraints])  # type: ignore

tests/test_generics.py

@@ -2639,6 +2639,7 @@ def test_serialize_unsubstituted_typevars_bound() -> None:
     class ErrorDetails(BaseModel):
         foo: str
 
+    # This version of `TypeVar` does not support `default` on Python <3.12
     ErrorDataT = TypeVar('ErrorDataT', bound=ErrorDetails)
 
     class Error(BaseModel, Generic[ErrorDataT]):
@@ -2696,6 +2697,68 @@ class MyErrorDetails(ErrorDetails):
     }
 
 
+def test_serialize_unsubstituted_typevars_bound_default_supported() -> None:
+    class ErrorDetails(BaseModel):
+        foo: str
+
+    # This version of `TypeVar` always support `default`
+    ErrorDataT = TypingExtensionsTypeVar('ErrorDataT', bound=ErrorDetails)
+
+    class Error(BaseModel, Generic[ErrorDataT]):
+        message: str
+        details: ErrorDataT
+
+    class MyErrorDetails(ErrorDetails):
+        bar: str
+
+    sample_error = Error(
+        message='We just had an error',
+        details=MyErrorDetails(foo='var', bar='baz'),
+    )
+    assert sample_error.details.model_dump() == {
+        'foo': 'var',
+        'bar': 'baz',
+    }
+    assert sample_error.model_dump() == {
+        'message': 'We just had an error',
+        'details': {
+            'foo': 'var',
+            'bar': 'baz',
+        },
+    }
+
+    sample_error = Error[ErrorDetails](
+        message='We just had an error',
+        details=MyErrorDetails(foo='var', bar='baz'),
+    )
+    assert sample_error.details.model_dump() == {
+        'foo': 'var',
+        'bar': 'baz',
+    }
+    assert sample_error.model_dump() == {
+        'message': 'We just had an error',
+        'details': {
+            'foo': 'var',
+        },
+    }
+
+    sample_error = Error[MyErrorDetails](
+        message='We just had an error',
+        details=MyErrorDetails(foo='var', bar='baz'),
+    )
+    assert sample_error.details.model_dump() == {
+        'foo': 'var',
+        'bar': 'baz',
+    }
+    assert sample_error.model_dump() == {
+        'message': 'We just had an error',
+        'details': {
+            'foo': 'var',
+            'bar': 'baz',
+        },
+    }
+
+
 @pytest.mark.parametrize(
     'type_var',
     [
@@ -2704,7 +2767,7 @@ class MyErrorDetails(ErrorDetails):
     ],
     ids=['default', 'constraint'],
 )
-def test_serialize_unsubstituted_typevars_bound(
+def test_serialize_unsubstituted_typevars_variants(
     type_var: Type[BaseModel],
 ) -> None:
     class ErrorDetails(BaseModel):