mypy sometimes confuses nested classes-in-functions which share the same name

[finite-state-machine]

Bug Report

In some circumstances, mypy will confuse two classes of the same name, even if they're declared in non-overlapping scopes.

To Reproduce

Check the following ~minimal example with mypy:

[mypy-play.net]

# from __future__ import annotations  # with or without makes no difference

def test_a() -> None:

    class SomeClass:  # line 5
        def some_method(self) -> None:
            _ = some_var  # line 7

    some_var = None  # not necessary for repro; stops error on line 7

def test_b() -> None:

    instance: SomeClass

    class SomeClass:  # line 15
        pass

    instance = SomeClass()
            # error: Incompatible types in assignment (expression
            #        has type "SomeClass@15", variable has type
            #        "SomeClass@5")  [assignment]

Expected Behavior

No errors.

Actual Behavior

The tool confuses the two classes of the same name (SomeClass), despite these being declared in entirely different scopes.

Bisection

The problem appears to have been introduced by 389a172 (which made its way into mypy 0.900):

389a1725c1df7fad4e2605360157c9f910cb502c is the first bad commit
commit 389a1725c1df7fad4e2605360157c9f910cb502c
Author: Jukka Lehtosalo <jukka.lehtosalo@iki.fi>
Date:   Thu May 6 14:32:36 2021 +0100

    Fix crash with nested NamedTuple in incremental mode (#10431)

    The name of the nested tuple type was inconsistent. Sometimes if was stored
    using the full name in the module symbol table.

    Also improve the internal API for creating classes to be less error-prone.

    Work on #7281.

 mypy/semanal.py                       | 10 +++++++---
 mypy/semanal_enum.py                  |  9 +++++----
 mypy/semanal_namedtuple.py            |  2 +-
 mypy/semanal_newtype.py               | 11 ++++++-----
 mypy/semanal_shared.py                |  2 +-
 mypy/semanal_typeddict.py             | 14 ++++++++------
 test-data/unit/check-incremental.test | 26 ++++++++++++++++++++++++++
 7 files changed, 54 insertions(+), 20 deletions(-)

Probably relates to #7281, #10431.

Your Environment

• Mypy version used: 1.9.0, 0.900, but not 0.800 (see: "bisection")
• Mypy command-line flags: none necessary
• Mypy configuration options from mypy.ini (and other config files): none necessary
• Python version used: 3.8, 3.12

[finite-state-machine]

(I'm going to mention @JukkaL, in no way as a criticism of their good work, but simply because they're most likely to be familiar with the code in question.)