Bytes formatting error gets produced during failed overload match

[JelleZijlstra]

Bug Report

On Python 3 formatting "b'abc'" with "{}" produces "b'abc'", not "abc" gets produced during bidirectional inference in a failed overload match.

To Reproduce

from typing import overload, Callable

@overload
def sub(pattern: str, repl: Callable[[str], str]) -> str: ...
@overload
def sub(pattern: bytes, repl: Callable[[bytes], bytes]) -> bytes: ...
def sub(pattern: object, repl: object) -> object:
    raise NotImplementedError

def better_snakecase(text: str) -> str:
    text = sub(r"([A-Z])([A-Z]+)([A-Z](?:[^A-Z]|$))", lambda match: f"{match}")
    return text

Expected Behavior

No errors

Actual Behavior

test_cases/stdlib/typing/pattern.py:11: error: On Python 3 formatting "b'abc'" with "{}" produces "b'abc'", not "abc"; use "{!r}" if this is desired behavior

Your Environment

• Mypy version used: master, 0.942
• Mypy command-line flags: none

---

Found in python/typeshed#7679, the repro case minified from some code in homeassistant.

[AlexWaygood]

This appears to have been fixed by a3abd36 (#12631), unless I did the git bisect wrong. I can't reproduce the error on the original example with a3abd36 checked out, but I can with the commit before it.

[hauntsaninja]

Hmm, this happened to me too with that exact same PR: #8814 (comment) . Since it's unexpected, seems worth understanding the mechanism / adding regression tests.

[AlexWaygood]

Oh, looks like it's actually not entirely unexpected: Jelle mentioned in #12631 (comment) that the PR was touching logic that impacted this bug. But we should definitely add a test, agreed.

[sebwills]

I'm getting the same error mypy_test.py:6: error: On Python 3 formatting "b'abc'" with "{}" produces "b'abc'", not "abc"; use "{!r}" if this is desired behavior [str-bytes-safe] with the following repro

import pandas as pd

df = pd.DataFrame({'A': [1]})

x = f"Hello {df.loc[0, 'A']}"

mypy: 1.0.0 (strict=True)

[sebwills]

Sorry, ignore my previous comment, it went away when I upgraded pandas-stubs

[erictraut]

This appears to have been fixed in mypy 0.960.

[AlexWaygood]

We were keeping this open so that we didn't forget to add a regression test, since this was a strange bug that was fixed somewhat by accident: #12665 (comment). But it looks like we forgot to anyway :)

I filed #15867 to add a regression test.