Assertion and checking functions

I'll just leave this here for potential future consideration as it came up on gitter's typing-dev channel. Typescript has something called assertion functions, which enables the type checker to understand that a function does not return if a certain condition is false. I think something similar, but more generalized, would be interesting, especially for unit test functions like assert_is_instance(), assert_true() etc.

def assert_positive(x: object) -> AssertsInstance["x", int]:
    assert isinstance(x, int) and x > 0

Related, checking functions could work like this:

def is_positive(x: object) -> ChecksInstance["x", int]:
    return isinstance(x, int) and x > 0

def my_func(x: Union[int, str]) -> None:
    if is_positive(x):
        # x must be an int
        ...
    else:
        # x can be an int or str
        ...

从链接的 TypeScript 3.7 assertion-functions 文档以及 issue 中的 assert_positive 和 is_positive 示例开始。由于没有指定 repository 文件或测试，首先定义通用断言和检查注解应如何表达类型收窄，然后记录已达成一致的行为和所需的测试覆盖范围。