You really need dedicated types for `int64` and something like `final`. Consider:

    class Foo:
      __slots__ = ("a", "b")
      a: int
      b: float

* a user can define subtype `class my_int(int)`, so you cannot optimize the layout of `class Foo`

* the builtin `int` and `float` are big-int like numbers, so operations on them are branchy and allocating.

and the fact that Foo is mutable and that `id(foo.a)` has to produce something complicates things further.

Maybe, but I quoted specific part I was replying to. TS has no impact on runtime performance of JS. Type hints in Python have no impact on runtime performance of Python (unless you try things like mypyc etc; actually, mypy provides `from mypy_extensions import i64`)

Therefore Python has no use for TS-like superset, because it already has facilities for static analysis with no bearing on runtime, which is what TS provides.