Support user defined generic field types in generic models. (#2554)

Work on #2465 (see comments)

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

changes/2465-daviskirk.md

@@ -0,0 +1 @@
+Support user defined generic field types in generic models.

pydantic/generics.py

@@ -172,7 +172,12 @@ def replace_types(type_: Any, type_map: Mapping[Any, Any]) -> Any:
             # If all arguments are the same, there is no need to modify the
             # type or create a new object at all
             return type_
-        if origin_type is not None and isinstance(type_, typing_base) and not isinstance(origin_type, typing_base):
+        if (
+            origin_type is not None
+            and isinstance(type_, typing_base)
+            and not isinstance(origin_type, typing_base)
+            and getattr(type_, '_name', None) is not None
+        ):
             # In python < 3.9 generic aliases don't exist so any of these like `list`,
             # `type` or `collections.abc.Callable` need to be translated.
             # See: https://www.python.org/dev/peps/pep-0585

tests/test_generics.py

@@ -1,6 +1,20 @@
 import sys
 from enum import Enum
-from typing import Any, Callable, ClassVar, Dict, Generic, List, Optional, Sequence, Tuple, Type, TypeVar, Union
+from typing import (
+    Any,
+    Callable,
+    ClassVar,
+    Dict,
+    Generic,
+    List,
+    Mapping,
+    Optional,
+    Sequence,
+    Tuple,
+    Type,
+    TypeVar,
+    Union,
+)
 
 import pytest
 from typing_extensions import Annotated, Literal
@@ -808,6 +822,30 @@ class Model(GenericModel, Generic[T]):
         assert replace_types(list[Union[str, list, T]], {T: int}) == list[Union[str, list, int]]
 
 
+@skip_36
+def test_replace_types_with_user_defined_generic_type_field():
+    """Test that using user defined generic types as generic model fields are handled correctly."""
+
+    T = TypeVar('T')
+    KT = TypeVar('KT')
+    VT = TypeVar('VT')
+
+    class GenericMapping(Mapping[KT, VT]):
+        pass
+
+    class GenericList(List[T]):
+        pass
+
+    class Model(GenericModel, Generic[T, KT, VT]):
+
+        map_field: GenericMapping[KT, VT]
+        list_field: GenericList[T]
+
+    assert replace_types(Model, {T: bool, KT: str, VT: int}) == Model[bool, str, int]
+    assert replace_types(Model[T, KT, VT], {T: bool, KT: str, VT: int}) == Model[bool, str, int]
+    assert replace_types(Model[T, VT, KT], {T: bool, KT: str, VT: int}) == Model[T, VT, KT][bool, int, str]
+
+
 @skip_36
 def test_replace_types_identity_on_unchanged():
     T = TypeVar('T')