MyPy not inferring type from assert statement.

Bug Report

MyPy is not inferring the right type after an assert statement.

To Reproduce

assert my_options is None or isinstance(my_options.options, CustomOptions)
return MyClass(
        options=my_options.options if my_options else None
)

my_options.options can be CustomOptions or AnotherCustomOptions, but MyClass supports only CustomOptions.

Actual Behavior

error: Argument "options" to "MyClass" has incompatible type "Union[CustomOptions, AnotherCustomOptions, None]"; expected "Optional[CustomOptions]"  [arg-type]

Your Environment

• Mypy version used: 1.3.0
• Mypy command-line flags: --config-file mypy.ini
• Mypy configuration options from mypy.ini (and other config files):

[mypy]
strict = true
plugins = sqlalchemy.ext.mypy.plugin
mypy_path = $MYPY_CONFIG_FILE_DIR/stubs/

• Python version used: 3.11.6

Start with the provided Python reproducer under mypy 1.3.0 and the stated strict configuration, then trace how the assert and conditional expression are analyzed. Add a regression test for this narrowing scenario and verify that mypy produces the intended type result without the reported incompatible-type error.