Formatter fails to parenthesize walrus expressions in function return types

[charliermarsh]

I saw this example in Black:

def this_is_so_dumb() -> (please := no): ...

def this_is_so_dumb(x) -> (please := no): ...

Right now, the formatter strips parentheses on the right-hand side, seemingly because optional_parentheses doesn't take required parentheses into account, unlike maybe_parenthesize_expression.

[charliermarsh]

@konstin - Do you know if it's possible to remove optional_parentheses, and replace its usages with maybe_parenthesize_expression? My experience from #6436 suggests that it might be possible, but might in turn require a new Parenthesize variant to support the "always parenthesize if the group breaks" behavior seen in that PR for the no-function-arguments case.

[charliermarsh]

@konstin - (Feel free to ignore, actually, I need to explore this more myself before I ask for your time.)