Add regression tests of inference of implicit None return

tests/functional/a/abstract/abstract_class_instantiated_py3.py

@@ -58,7 +58,7 @@ def __contains__(self, _):
 
 class Sizable(Structure):
     def __len__(self):
-        pass
+        return 42
 
 class Hashable(Structure):
     __hash__ = 42

tests/functional/a/abstract/abstract_method_py3.py

@@ -75,7 +75,7 @@ def __contains__(self, _):
 # +1: [abstract-method, abstract-method, abstract-method]
 class Sizable(Structure):
     def __len__(self):
-        pass
+        return 42
 
 
 # +1: [abstract-method, abstract-method, abstract-method]

tests/functional/r/regression/regression_implicit_none_with_no_return.py

@@ -0,0 +1,18 @@
+# pylint: disable=missing-docstring,pointless-statement,invalid-name,no-self-use
+
+class A:
+    def func(self):
+        print('hello')
+
+    @property
+    def index(self):
+        print('world')
+
+
+# Since these are not assigned anywhere, assignment-from-none etc.
+# in typecheck does not warn
+
+a = A()
+# double call is workaround for #4426
+a.func()()  # [not-callable]
+[1, 2, 3][a.index]  # [invalid-sequence-index]

tests/functional/r/regression/regression_implicit_none_with_no_return.txt

@@ -0,0 +1,2 @@
+not-callable:17:0::a.func() is not callable
+invalid-sequence-index:18:0::Sequence index is not an int, slice, or instance with __index__