Checking elements of a `list[str | None]` after discarding the possibility of `None`

Bug Report

str_none: list[str | None]
only_str: list[str]
for i in range(len(sequence)):
    if sequence[i] is not None:
        only_str.append(sequence[i]) # Here mypy raises an error due to "incompatible types"

The code above raises the following error Argument 1 to "append" of "list" has incompatible type "str | None"; expected "str".

I've found the following verbose workaround but to my taste its :

str_none: list[str | None]
only_str: list[str]
for i in range(len(sequence)):
    element = sequence[i]
    if element is not None:
        only_str.append(element)

To Reproduce

See code example above.

Expected Behavior

I'm expecting mypy to be able to predict future annotations of sequences with multiple types possible after discarding a specific type in a condition.

Actual Behavior

All is included above.

Your Environment

• Mypy version used: 1.5.1
• Mypy command-line flags: both --strict and none
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.11.5

Comienza ejecutando el reproductor mínimo del issue con mypy 1.5.1 y, después, rastrea el comportamiento del estrechamiento de tipos para el acceso indexado a secuencias. Se considerará terminado cuando el ejemplo acepte el elemento cuyo tipo se ha estrechado sin la solución alternativa verbosa que usa una variable local, con una prueba de regresión que cubra el caso descrito.