Fix issue with in-place modification of FieldInfo

changes/4067-adriangb.md

@@ -0,0 +1 @@
+fix in-place modification of FieldInfo that made it impossible to use PEP 593 type aliases

pydantic/fields.py

@@ -1,3 +1,4 @@
+import copy
 from collections import Counter as CollectionCounter, defaultdict, deque
 from collections.abc import Hashable as CollectionsHashable, Iterable as CollectionsIterable
 from typing import (
@@ -446,6 +447,7 @@ def _get_field_info(
                 raise ValueError(f'cannot specify multiple `Annotated` `Field`s for {field_name!r}')
             field_info = next(iter(field_infos), None)
             if field_info is not None:
+                field_info = copy.copy(field_info)
                 field_info.update_from_config(field_info_from_config)
                 if field_info.default is not Undefined:
                     raise ValueError(f'`Field` default cannot be set in `Annotated` for {field_name!r}')

tests/test_annotated.py

@@ -1,3 +1,5 @@
+from typing import List
+
 import pytest
 from typing_extensions import Annotated
 
@@ -132,3 +134,21 @@ class Config:
     assert Foo.schema(by_alias=True)['properties'] == {
         'a': {'title': 'A', 'description': 'descr', 'foobar': 'hello', 'type': 'integer'},
     }
+
+
+def test_annotated_alias() -> None:
+    # https://github.com/samuelcolvin/pydantic/issues/2971
+
+    StrAlias = Annotated[str, Field(max_length=3)]
+    IntAlias = Annotated[int, Field(default_factory=lambda: 2)]
+
+    Nested = Annotated[List[StrAlias], Field(description='foo')]
+
+    class MyModel(BaseModel):
+        a: StrAlias = 'abc'
+        b: StrAlias
+        c: IntAlias
+        d: IntAlias
+        e: Nested
+
+    assert MyModel(b='def', e=['xyz']) == MyModel(a='abc', b='def', c=2, d=2, e=['xyz'])