mypy does not understand that in multiple assignment any involved expressions keep their type until the end of all assignments

Example:

x: tuple[int, int]|int = (1, 2)
if isinstance(x, tuple):
    x, y = x[0], x[1]

main.py:3: error: Value of type "int" is not indexable [index]
Found 1 error in 1 file (checked 1 source file)

Another example I ran across (in an experimental version of checker.py I'm working on) involved two different objects, no tuples:

mypy/checker.py:7737: error: "str" has no attribute   "code"; maybe "encode"?  [attr-defined]
                msg, code = msg.value, msg.code
                                       ^~~~~~~~

These work at runtime so it's a problem with mypy.

Start by running mypy on the two examples in the issue, using main.py for the tuple case and the reported checker.py line 7737 for the second case. Trace how narrowing is preserved across multiple assignment, then verify that both examples type-check without weakening unrelated diagnostics.