Seemingly correct type narrowing, but still incompatible type

[hwong557]

Bug Report

Mypy seems to type narrow correctly, but still throws an error when trying to add to a set.

To Reproduce

Gist URL: https://gist.github.com/c10befca044d78713b96a9706ac355b2
Playground URL: https://mypy-play.net/?mypy=master&python=3.10&gist=c10befca044d78713b96a9706ac355b2

Expected Behavior

No error.

Actual Behavior

main.py:15: error: Argument 1 to "add" of "set" has incompatible type "Data"; expected "Data"
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 0.990+dev.780534b13722b7b0422178c049a1cbbf4ea4255b (compiled: no)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10.7

[JelleZijlstra]

Here's an interesting variation: https://mypy-play.net/?mypy=master&python=3.10&gist=be6708082fc74cb9e2577840877b1741. If you declare a variable as of type Data directly, its revealed type is "Tuple[builtins.str, builtins.int, fallback=__main__.Data]" instead of __main__.Data.

[sterliakov]

This is resolved (fails up to and including 1.6.1, works in 1.7.0 through current master). Bisects to seemingly unrelated #249f3f (PR #16053)

[hauntsaninja]

Thanks, yeah I guess subtyping changes in that PR help

[ilevkivskyi]

Actually it looks like #16053 introduced a subtle bug in subtyping, this is why it fixed this issue. I will at fixing myself soon, I am not going to re-open this, as there is a conveniently open duplicate #15299, that also looks fixed because of the subtyping bug.