Mypy incorrectly gives an error for a ternary return statement

[johndoknjas]

Bug Report

Hi, I was using mypy and I think I've noticed a bug. For a function that returns either List[str] or List[List[str]], mypy gives an error if the return type is decided as a ternary. However, if the return type is decided with an 'if-else' instead, there are no problems. I also tested a similar function, except where the return type is either a float or str. In that case, mypy had no problems with the ternary usage.

To Reproduce

from typing import Union, List

def get_str_float(b: bool) -> Union[str, float]:
    return 8 if b else '8' # mypy correctly gives no error here

def get_list(b: bool) -> Union[List[str], List[List[str]]]:
    return ['a'] if b else [['a']] # mypy incorrectly gives an error
    """
    Mypy gives no error if I replace the above line with the following:
    if b:
        return ['a']
    else:
        return [['a']]
    """

def main():
    print(get_list(True))
    print(get_str_float(True))

if __name__ == "__main__":
    main()

Expected Behavior

No errors

Actual Behavior

main.py:7: error: List item 0 has incompatible type "List[str]"; expected "str" [list-item]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 1.3.0 (compiled: yes)
• Mypy command-line flags: none - just ran mypy main.py
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.11.0
• OS: Windows 11