Fix enum convertor returning None for already-converted values

tests/test_type_conversion.py

@@ -85,6 +85,15 @@ class SomeEnum(Enum):
     THREE = "three"
 
 
+def test_enum_convertor_already_converted():
+    """If the value is already an enum member, the convertor should return it as-is."""
+    from typer.main import generate_enum_convertor
+
+    convertor = generate_enum_convertor(SomeEnum)
+    result = convertor(SomeEnum.ONE)
+    assert result is SomeEnum.ONE
+
+
 @pytest.mark.parametrize(
     "type_annotation",
     [list[Path], list[SomeEnum], list[str]],

typer/main.py

@@ -1446,6 +1446,8 @@ def generate_enum_convertor(enum: type[Enum]) -> Callable[[Any], Any]:
 
     def convertor(value: Any) -> Any:
         if value is not None:
+            if isinstance(value, enum):
+                return value
             val = str(value)
             if val in val_map:
                 key = val_map[val]