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

Führe zunächst den minimalen Reproducer aus dem Issue mit mypy 1.5.1 aus und verfolge anschließend das Verhalten der Typ-Eingrenzung beim indizierten Zugriff auf Sequenzen. Fertig ist die Arbeit, wenn das Beispiel das eingegrenzte Element ohne den ausführlichen Workaround mit einer lokalen Variable akzeptiert und ein Regressionstest den gemeldeten Fall abdeckt.