TypeGuard is incompatible with exhaustiveness checking

[antonagestam]

Bug Report

Narrowing with a TypeGuard function doesn't work in conjunction with exhaustiveness checking using assert_never().

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.11&gist=b6bc8609a83ff0538459d658c74f23ea

Expected Behavior

In the example, I expected the TypeGuard function to work no differently from the inlined type checks.

Actual Behavior

When using TypeGuard, the variable seems to be properly narrowed (revealed_type() within the branch is identical), but it seems somehow the result isn't recorded identically as the inlined checks. In the function using TypeGuard, the exhaustiveness check is not passing.

Your Environment

• Mypy version used: 1.3.0
• Mypy command-line flags: -
• Mypy configuration options from mypy.ini (and other config files): -
• Python version used: 3.11

[erictraut]

Mypy is correct here. TypeGuard does not perform narrowing in the negative (else) case. For the reasoning behind this, you can refer to PEP 647. There has been some discussion of adding a variant called StrictTypeGuard that is more limited than TypeGuard but allows negative type narrowing. This would require a new PEP, and it hasn't made it beyond the discussion phase yet.

[AlexWaygood]

What Eric said

[antonagestam]

Ah my bad, thanks for explaining! 🙏