[1.20] Regression on Mapping equality type narrowing

[Dreamsorcerer]

Bug Report

Report requested from #20726 (comment)

To Reproduce

from collections.abc import Mapping
x: Mapping[str, str]
assert x == {"a": "b"}
x["a"] = "foo"

Expected Behavior

mypy 1.19 produces an index error.
mypy 1.20 seems to incorrectly narrow the type to a dict and then allows the assignment, which may not actually work.

[ilevkivskyi]

Too late LOL, this is already fixed on master.