Functions with Optional[...] return annotations should not need all return statements

[retnikt]

Please provide more information to help us understand the issue:

• Are you reporting a bug, or opening a feature request? Bug
• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

def foo(bar: int) -> Optional[int]
    if bar == 3:
        return 6

• What is the actual behavior/output?

error: Missing return statement

• What is the behavior/output you expect?
  No error.
  If the function has a Optional return type, then surely it does not need a return statement everywhere because it might return None, and not returning means it implicitly returns None
• What are the versions of mypy and Python you are using? mypy 0.780, CPython 3.8.3
  Do you see the same issue after installing mypy from Git master? Yes
• What are the mypy flags you are using? (For example --strict-optional): None

According to the docs:

--no-warn-no-return

By default, mypy will generate errors when a function is missing return statements in some execution paths. The only exceptions are when:

   The function has a `None` or `Any` return type
   The function has an empty body or a body that is just ellipsis (`...`). Empty functions are often used for abstract methods.

Passing in --no-warn-no-return will disable these error messages in all cases.

So an exception for Optional should be added too (and Unions that contain None).

[oakkitten]

duplicate of #3974

[JukkaL]

Another duplicate is #7511.