Fix accidentally disallowing empty returns in strict-optional (#1981)

Author: ddfisher

mypy/checker.py

@@ -1453,10 +1453,7 @@ def visit_return_stmt(self, s: ReturnStmt) -> Type:
                 if (self.function_stack[-1].is_generator and isinstance(return_type, AnyType)):
                     return None
 
-                if isinstance(return_type, Void):
-                    return None
-
-                if isinstance(return_type, AnyType):
+                if isinstance(return_type, (Void, NoneTyp, AnyType)):
                     return None
 
                 if self.typing_mode_full():

test-data/unit/check-optional.test

@@ -298,3 +298,11 @@ def g(x: Optional[int]) -> int:
 x = f()  # E: Function does not return a value
 f() + 1  # E: Function does not return a value
 g(f())  # E: Function does not return a value
+
+[case testEmptyReturn]
+def f() -> None:
+    return
+
+[case testReturnNone]
+def f() -> None:
+    return None