False positive following case None in match statement (missed narrowing?)

[mthuurne]

Bug Report

In match block with a case None:, mypy considers values captured by subsequent cases to still have None as a possible type.

To Reproduce

Check the following code using mypy:

def f(obj: object) -> str | None:
    return str(obj) if obj else None

def g(obj: object) -> str:
    match f(obj):
        case None:
            return "-"
        case text:
            return text

Expected Behavior

No errors would be reported: a case that matches the literal None can be treated as matching NoneType.

Actual Behavior

match_nonetype.py:9: error: Incompatible return value type (got "Optional[str]", expected "str")  [return-value]
                return text
                       ^

Your Environment

• Python version used: 3.10.4
• Operating system and version: openSUSE Linux Tumbleweed
• Mypy version used: 0.961 (compiled: yes)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files):

disallow_any_generics=True
disallow_incomplete_defs=True
disallow_untyped_defs=True
namespace_packages=True
no_implicit_optional=True
no_implicit_reexport=True
pretty=True
show_error_codes=True
strict_equality=True
warn_no_return=True
warn_redundant_casts=True
warn_return_any=True
warn_unreachable=True
warn_unused_configs=True
warn_unused_ignores=True

[mthuurne]

The problem can be worked around by assigning the result of the function call to a local variable:

def f(obj: object) -> str | None:
    return str(obj) if obj else None

def g(obj: object) -> str:
    x = f(obj)
    match x:
        case None:
            return "-"
        case text:
            return text

So this might be a variation of #12998.

[dset0x]

For what it's worth, it appears that the following is also currently accepted by mypy:

    match _ := f(obj):
        case None:
            return "-"
        case text:
            return text

[erictraut]

This is a duplicate of #12998.