Narrow types using overload information

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
25/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
compilers, devtools

Research direction

Start with the overload and NoReturn example in this issue, then review mypy’s existing handling of overloads and NoReturn. Define how overload information should narrow variables and conditional branches, including correlated arguments and generic compatibility, and validate the behavior with focused type-checking cases.

Written by the indexing model from the issue text.

Description

feature needs discussion topic-overloads

mypy can use NoReturn to narrow types of variables, thus enabling us to express some kinds of guards, in the sense of #1203 / #2357, without using any dedicated mechanism. For example

from typing import Union, NoReturn, overload

@overload
def assert_int(x: int) -> None: ...
@overload
def assert_int(x: str) -> NoReturn: ...

def assert_int(x):
    assert isinstance(x, int)

y: Union[int, str]
assert_int(y)
reveal_type(y)  # should be: int; currently Union[int, str]

The same technique could be used to narrow types in conditionals, using only "always true" / "always false" types (do we already have such things? maybe it requires singleton True False literal-types).

Note that unlike simple isinstance, this can also be used to correlate two different arguments, test for generic compatibility, etc; it is the responsibility of the programmer to have an actual implementation.

Dominant language
Python
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Merged PRs (30d)
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