Fix descriptor overload selection (#18868)

Fixes #15921

I know there were previously concerns about performance of
`check_self_arg()`, but note that the code path where I add it only
affects descriptors and `__getattr__`/`__setattr__`, so I think it
should be OK.

Author: ilevkivskyi

mypy/checkmember.py

@@ -42,6 +42,7 @@
     erase_to_bound,
     freeze_all_type_vars,
     function_type,
+    get_all_type_vars,
     get_type_vars,
     make_simplified_union,
     supported_self_type,
@@ -604,7 +605,10 @@ def analyze_member_var_access(
             setattr_meth = info.get_method("__setattr__")
             if setattr_meth and setattr_meth.info.fullname != "builtins.object":
                 bound_type = analyze_decorator_or_funcbase_access(
-                    defn=setattr_meth, itype=itype, name=name, mx=mx.copy_modified(is_lvalue=False)
+                    defn=setattr_meth,
+                    itype=itype,
+                    name="__setattr__",
+                    mx=mx.copy_modified(is_lvalue=False),
                 )
                 typ = map_instance_to_supertype(itype, setattr_meth.info)
                 setattr_type = get_proper_type(expand_type_by_instance(bound_type, typ))
@@ -1031,7 +1035,16 @@ def f(self: S) -> T: ...
             selfarg = get_proper_type(item.arg_types[0])
             # This level of erasure matches the one in checker.check_func_def(),
             # better keep these two checks consistent.
-            if subtypes.is_subtype(dispatched_arg_type, erase_typevars(erase_to_bound(selfarg))):
+            if subtypes.is_subtype(
+                dispatched_arg_type,
+                erase_typevars(erase_to_bound(selfarg)),
+                # This is to work around the fact that erased ParamSpec and TypeVarTuple
+                # callables are not always compatible with non-erased ones both ways.
+                always_covariant=any(
+                    not isinstance(tv, TypeVarType) for tv in get_all_type_vars(selfarg)
+                ),
+                ignore_pos_arg_names=True,
+            ):
                 new_items.append(item)
             elif isinstance(selfarg, ParamSpecType):
                 # TODO: This is not always right. What's the most reasonable thing to do here?
@@ -1164,6 +1177,7 @@ def analyze_class_attribute_access(
             def_vars = set(node.node.info.defn.type_vars)
             if not node.node.is_classvar and node.node.info.self_type:
                 def_vars.add(node.node.info.self_type)
+            # TODO: should we include ParamSpec etc. here (i.e. use get_all_type_vars)?
             typ_vars = set(get_type_vars(t))
             if def_vars & typ_vars:
                 # Exception: access on Type[...], including first argument of class methods is OK.
@@ -1405,6 +1419,6 @@ def analyze_decorator_or_funcbase_access(
     """
     if isinstance(defn, Decorator):
         return analyze_var(name, defn.var, itype, mx)
-    return bind_self(
-        function_type(defn, mx.chk.named_type("builtins.function")), original_type=mx.self_type
-    )
+    typ = function_type(defn, mx.chk.named_type("builtins.function"))
+    typ = check_self_arg(typ, mx.self_type, defn.is_class, mx.context, name, mx.msg)
+    return bind_self(typ, original_type=mx.self_type, is_classmethod=defn.is_class)

test-data/unit/check-classes.test

@@ -3135,7 +3135,8 @@ from typing import Any
 class Test:
     def __setattr__() -> None: ...  # E: Method must have at least one argument. Did you forget the "self" argument? # E: Invalid signature "Callable[[], None]" for "__setattr__"
 t = Test()
-t.crash = 'test'  # E: "Test" has no attribute "crash"
+t.crash = 'test'  # E: Attribute function "__setattr__" with type "Callable[[], None]" does not accept self argument \
+                  # E: "Test" has no attribute "crash"
 
 class A:
     def __setattr__(self): ...  # E: Invalid signature "Callable[[A], Any]" for "__setattr__"
@@ -8648,3 +8649,34 @@ class C(B):
     def meth(self) -> None:
         def cb() -> None:
             self.x: int = 1  # E: Incompatible types in assignment (expression has type "int", base class "B" defined the type as "str")
+
+[case testOverloadedDescriptorSelected]
+from typing import Generic, TypeVar, Any, overload
+
+T_co = TypeVar("T_co", covariant=True)
+class Field(Generic[T_co]):
+    @overload
+    def __get__(self: Field[bool], instance: None, owner: Any) -> BoolField: ...
+    @overload
+    def __get__(self: Field[int], instance: None, owner: Any) -> NumField: ...
+    @overload
+    def __get__(self: Field[Any], instance: None, owner: Any) -> AnyField[T_co]: ...
+    @overload
+    def __get__(self, instance: Any, owner: Any) -> T_co: ...
+
+    def __get__(self, instance: Any, owner: Any) -> Any:
+        pass
+
+class BoolField(Field[bool]): ...
+class NumField(Field[int]): ...
+class AnyField(Field[T_co]): ...
+class Custom: ...
+
+class Fields:
+    bool_f: Field[bool]
+    int_f: Field[int]
+    custom_f: Field[Custom]
+
+reveal_type(Fields.bool_f)  # N: Revealed type is "__main__.BoolField"
+reveal_type(Fields.int_f)  # N: Revealed type is "__main__.NumField"
+reveal_type(Fields.custom_f)  # N: Revealed type is "__main__.AnyField[__main__.Custom]"