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

Start by running the minimal reproducer from the issue with mypy 1.5.1 and then trace the type-narrowing behavior for indexed sequence access. Done means the example accepts the narrowed element without the verbose local-variable workaround, with a regression test covering the reported case.