Custom type narrowing with a special decorator

[gantsevdenis]

This is surely not meant to be merged.. but just a small proposal?
I think the narrow_cast (or whatever it's called) should be inside typing and not mypy.
Having that decorator, type narrowing with user-defined functions is trivial (cf tests) and looks something like

@narrow_cast(str)  # mypy will narrow type to 'str' if this returns True
def is_fizz_buzz(foo):
    return foo in ['fizz', 'buzz']

def foobar(foo: Union[str, float]):
    if foo in ['fizz', 'buzz']:
        reveal_type(foo)  # N: Revealed type is 'Union[builtins.str, builtins.float]'
    if is_fizz_buzz(foo):
        reveal_type(foo)  # N: Revealed type is 'builtins.str'

[ilevkivskyi]

This looks too ad-hoc TBH. I would rather implement support for type-guards using overloads, see #4063 and #5206. This will not require any new special forms and will be more flexible.

[DustinWehr]

@ilevkivskyi so is your concern that a more-general feature that comes later (but how much later?) would make this (seemingly) simple one obsolete? That doesn't seem like a big deal, especially if the documentation for @narrow_cast expressed that expectation, or if we later provided a little converter script that converts source code containing @narrow_cast to use the more-general mechanism.

@gantsevdenis if you got this working, I'd be interested enough to help maintain a fork.