support overwriting dunder attributes of BaseModel instances (#3907)

* support overwriting dunder attributes of `BaseModel` instances

closes #3777

* suggestion from @adriangb

Co-authored-by: Samuel Colvin <s@muelcolvin.com>

tests/test_main.py

@@ -19,6 +19,7 @@ from typing import (
 from uuid import UUID, uuid4
 
 import pytest
+from typing_extensions import Annotated
 
 from pydantic import (
     BaseConfig,
@@ -2174,6 +2175,19 @@ def test_new_union_origin():
     }
 
 
+def test_annotated_class():
+    class PydanticModel(BaseModel):
+        foo: str = '123'
+
+    PydanticAlias = Annotated[PydanticModel, 'bar baz']
+
+    pa = PydanticAlias()
+    assert isinstance(pa, PydanticModel)
+    pa.__doc__ = 'qwe'
+    assert repr(pa) == "PydanticModel(foo='123')"
+    assert pa.__doc__ == 'qwe'
+
+
 @pytest.mark.parametrize(
     'ann',
     [Final, Final[int]],