Capture the behavior of descriptors that don't define __get__

Author: cpennington

mypy/checkmember.py

@@ -209,10 +209,6 @@ def analyze_member_var_access(name: str, itype: Instance, info: TypeInfo,
                                 return dunder_set_type.arg_types[1]
                         else:
                             msg.fail("Can't set a data descriptor with no __set__ method", node)
-                    else:
-                        # TODO: Capture setting a non-data descriptor to infer types
-                        # later
-                        return AnyType()
                 else:
                     # this is a descriptor, use the output type of the __get__ method
                     if dunder_get:
@@ -227,12 +223,6 @@ def analyze_member_var_access(name: str, itype: Instance, info: TypeInfo,
                             return dunder_get_type.ret_type
                         else:
                             return AnyType()
-                    else:
-                        # TODO: Per the docs: "If [a descriptor] does not define __get__(),
-                        # then accessing the attribute will return the descriptor object
-                        # itself unless there is a value in the object’s instance dictionary".
-                        # Can we actually capture that during type inference?
-                        return AnyType()
 
         return analyze_var(name, v, itype, info, node, is_lvalue, msg, not_ready_callback)
     elif isinstance(v, FuncDef):

test-data/unit/check-classes.test

@@ -898,8 +898,6 @@ a.f.x # E: "int" has no attribute "x"
 
 
 [case testAccessingNonDataDescriptor]
-import typing
-
 class D():
     def __get__(self, instance: Any, owner: Any) -> str:
         return 'some string'
@@ -911,8 +909,6 @@ a = A() # type: A
 reveal_type(a.f)  # E: Revealed type is 'builtins.str'
 
 [case testSettingDataDescriptor]
-import typing
-
 class D():
     def __get__(self, instance: Any, owner: Any) -> str:
         return 'some string'
@@ -927,6 +923,22 @@ a = A() # type: A
 a.f = ''
 a.f = 1 # E: Incompatible types in assignment (expression has type "int", variable has type "str")
 
+[case testReadingDescriptorWithoutDunderGet]
+from typing import Union
+
+class D():
+    def __set__(self, instance: Any, value: str) -> None:
+        pass
+
+class A:
+    f = D()  # type: Union[D, str]
+
+    def __init__(self):
+        self.f = 'some string'  # type: str
+
+a = A() # type: A
+reveal_type(a.f)  # E: Revealed type is 'Union[__main__.D, builtins.str]'
+
 
 -- Multiple inheritance, non-object built-in class as base
 -- -------------------------------------------------------