fix: incorrect returned type of access descriptors on unions of types (#16604)

Fixes #16603

This change maps over union types when determining the types of access
descriptors. Previously, the because [this
conditional](https://github.com/md384/mypy/blob/c2a55afcef32ecb11a4c76c4c79539f6ba36d55c/mypy/checkmember.py#L697-L701)
would fall through to the `else` case because instance type was not a
singular `TypeType` (it was a Union), so we'd end up with an instance
value being passed to `__get__` instead of `None`.

Author: md384

mypy/checkmember.py

@@ -123,6 +123,7 @@ def copy_modified(
         messages: MessageBuilder | None = None,
         self_type: Type | None = None,
         is_lvalue: bool | None = None,
+        original_type: Type | None = None,
     ) -> MemberContext:
         mx = MemberContext(
             self.is_lvalue,
@@ -142,6 +143,8 @@ def copy_modified(
             mx.self_type = self_type
         if is_lvalue is not None:
             mx.is_lvalue = is_lvalue
+        if original_type is not None:
+            mx.original_type = original_type
         return mx
 
 
@@ -644,6 +647,16 @@ def analyze_descriptor_access(descriptor_type: Type, mx: MemberContext) -> Type:
         return make_simplified_union(
             [analyze_descriptor_access(typ, mx) for typ in descriptor_type.items]
         )
+    elif isinstance(instance_type, UnionType):
+        # map over the instance types
+        return make_simplified_union(
+            [
+                analyze_descriptor_access(
+                    descriptor_type, mx.copy_modified(original_type=original_type)
+                )
+                for original_type in instance_type.items
+            ]
+        )
     elif not isinstance(descriptor_type, Instance):
         return orig_descriptor_type
 

test-data/unit/check-unions.test

@@ -1220,3 +1220,41 @@ nc: Union[Container[str], int]
 'x' in nc  # E: Unsupported right operand type for in ("Union[Container[str], int]")
 [builtins fixtures/tuple.pyi]
 [typing fixtures/typing-full.pyi]
+
+[case testDescriptorAccessForUnionOfTypes]
+from typing import overload, Generic, Any, TypeVar, List, Optional, Union, Type
+
+_T_co = TypeVar("_T_co", bound=Any, covariant=True)
+
+class Mapped(Generic[_T_co]):
+    def __init__(self, value: _T_co):
+        self.value = value
+
+    @overload
+    def __get__(
+        self, instance: None, owner: Any
+    ) -> List[_T_co]:
+        ...
+
+    @overload
+    def __get__(self, instance: object, owner: Any) -> _T_co:
+        ...
+
+    def __get__(
+        self, instance: Optional[object], owner: Any
+    ) -> Union[List[_T_co], _T_co]:
+        return self.value
+
+class A:
+    field_1: Mapped[int] = Mapped(1)
+    field_2: Mapped[str] = Mapped('1')
+
+class B:
+    field_1: Mapped[int] = Mapped(2)
+    field_2: Mapped[str] = Mapped('2')
+
+mix: Union[Type[A], Type[B]] = A
+reveal_type(mix)  # N: Revealed type is "Union[Type[__main__.A], Type[__main__.B]]"
+reveal_type(mix.field_1)  # N: Revealed type is "builtins.list[builtins.int]"
+reveal_type(mix().field_1)  # N: Revealed type is "builtins.int"
+[builtins fixtures/list.pyi]