type assertions

[KotlinIsland]

def f(x: object) -> TypeAssertion[int]:  # demarcation is tentative, maybe `Asserts[x is int]`
    assert isinstance(x, int)
a: object
f(a)
reveal_type(a)  # int is

Notes:

• Would want to be able to provide an assertion and a return type

  def f(x: object) -> Asserts[x is int] & str: ...
  

• Would want to be able to do 'truthy' assertions / invariants

  def asserts(x: object) -> Asserts[x] ...
  a: object
  asserts(isinstance(a, bool))
  reveal_type(a)  # bool