Guard clause prevents `match` from seeing that all paths return

[bkovitz]

Bug Report

In a match statement in which all cases include a return statement, mypy 0.942 falsely reports "Missing return statement [return]" if at least one case includes a guard clause.

To Reproduce

def func(e) -> int:
    match e:
        case x if True:
            return x
        case _:
            return 0

Or even more simply:

def func(e) -> int:
    match e:
        case _ if True:
            return 0

Expected Behavior

This code should type-check.

Actual Behavior

$ mypy --python-version 3.10 --show-error-codes x5.py
x5.py:1: error: Missing return statement  [return]
Found 1 error in 1 file (checked 1 source file)

Notes

If we change the first case to something that does not include a guard clause, like this:

def func(e) -> int:
    match e:
        case (x, _):
            return x
        case _:
            return 0

then mypy returns no errors.

Environment

• Mypy version used: 0.942
• Mypy command-line flags: --python-version 3.10 --show-error-codes
• Mypy configuration options from mypy.ini (and other config files): (none)
• Python version used: 3.10.2
• Operating system and version: Ubuntu 18.04

[hauntsaninja]

Opened a PR that should fix this. Are any of the people here encountering this in open source projects? I could use some more match statement coverage in https://github.com/hauntsaninja/mypy_primer

[hauntsaninja]

I've merged the fix, but if your project is open source, please let me know!