Fix descriptor/Union interactions (#5039)

Fixes #4943

Author: msullivan

mypy/checkexpr.py

@@ -1278,7 +1278,13 @@ def analyze_descriptor_access(self, instance_type: Type, descriptor_type: Type,
         Return:
             The return type of the appropriate ``__get__`` overload for the descriptor.
         """
-        if not isinstance(descriptor_type, Instance):
+        if isinstance(descriptor_type, UnionType):
+            # Map the access over union types
+            return UnionType.make_simplified_union([
+                self.analyze_descriptor_access(instance_type, typ, context)
+                for typ in descriptor_type.items
+            ])
+        elif not isinstance(descriptor_type, Instance):
             return descriptor_type
 
         if not descriptor_type.type.has_readable_member('__get__'):

test-data/unit/check-classes.test

@@ -1495,6 +1495,20 @@ a = A()
 a.f = 1
 reveal_type(a.f)  # E: Revealed type is 'builtins.str'
 
+[case testDescriptorGetUnion]
+from typing import Any, Union
+class String:
+    def __get__(self, inst: Any, owner: Any) -> str:
+        return ''
+
+class A:
+    attr: str
+
+class B:
+    attr = String()
+
+def foo(x: Union[A, B]) -> None:
+    reveal_type(x.attr)  # E: Revealed type is 'builtins.str'
 
 -- _promote decorators
 -- -------------------