Add tests of generic descriptors

Author: cpennington

test-data/unit/check-classes.test

@@ -939,6 +939,49 @@ class A:
 a = A() # type: A
 reveal_type(a.f)  # E: Revealed type is 'Union[__main__.D, builtins.str]'
 
+[case testAccessingGenericNonDataDescriptor]
+from typing import TypeVar, Type, Generic
+
+T = TypeVar('T')
+V = TypeVar('V')
+class D(Generic[T, V]):
+    def __init__(self, value: V) -> None:
+        self.value = value
+    def __get__(self, instance: T, owner: Type[T]) -> V:
+        return self.value
+
+class A:
+    f = D(10)  # type: D[A, int]
+    g = D('10')  # type: D[A, str]
+
+a = A() # type: A
+reveal_type(a.f)  # E: Revealed type is 'builtins.int*'
+reveal_type(a.g)  # E: Revealed type is 'builtins.str*'
+
+[case testSettingGenericDataDescriptor]
+from typing import TypeVar, Type, Generic
+
+T = TypeVar('T')
+V = TypeVar('V')
+class D(Generic[T, V]):
+    def __init__(self, value: V) -> None:
+        self.value = value
+    def __get__(self, instance: T, owner: Type[T]) -> V:
+        return self.value
+    def __set__(self, instance: T, value: V) -> None:
+        pass
+
+class A:
+    f = D(10)  # type: D[A, int]
+    g = D('10')  # type: D[A, str]
+
+a = A() # type: A
+a.f = 1
+a.f = '' # E: Incompatible types in assignment (expression has type "str", variable has type "int")
+a.g = ''
+a.g = 1 # E: Incompatible types in assignment (expression has type "int", variable has type "str")
+
+
 
 -- Multiple inheritance, non-object built-in class as base
 -- -------------------------------------------------------