[1.20 regression] unreachable regression with generic Callable

Bug Report

Since mypy 1.20.0, a comparison between 2 generic callables may cause "Statement is unreachable"

To Reproduce

from typing import Callable

def identity[T](x: T) -> T:
    return x

def msg[T,S](cmp_property: Callable[[T], S]) -> None:
    if cmp_property == identity:
        return

Expected Behavior

Mypy should not report any error.

Actual Behavior

scpytools/kk.py: note: In function "msg":
scpytools/kk.py:10: error: Statement is unreachable  [unreachable]
            return
            ^~~~~~

Your Environment

Mypy version used: mypy 1.20.0 (compiled: yes)
Mypy command-line flags: mypy --warn-unreachable
Mypy configuration options from mypy.ini (and other config files): No config files
Python version used: 3.14.2

Thanks, changes in 1.20 surface a pre-existing deficiency in is_overlapping_types, will look more...

mypy 1.20: There is a bug in the analysis of a check for equality of …

One idea for a fix is this:

--- a/mypy/meet.py
+++ b/mypy/meet.py
@@ -548,6 +551,14 @@ def is_overlapping_types(
             is_proper_subtype=False,
             ignore_pos_arg_names=not overlap_for_overloads,
             allow_partial_overlap=True,
+        ) or is_callable_compatible(
+            right,
+            left,
+            is_compat=_is_overlapping_types,
+            is_proper_subtype=False,
+            ignore_pos_arg_names=not overlap_for_overloads,
+            check_args_covariantly=True,
+            allow_partial_overlap=True,
         )

This is nice because it obviously solves any other symmetry issues, but maybe it is a bandaid over some deeper issue

Okay, it looks like this is known and we do something similar elsewhere, see e.g. https://github.com/python/mypy/pull/5476/changes#diff-f96a2d6138bc6cdf2a07c4d37f6071cc25c1631afc107e277a28d5b59fc0ef04R3755 so I will open the PR

Fix is_overlapping_types for generic callables

Fix is_overlapping_types for generic callables (#21208)

Mypy: Removed temporary "type: ignore" that I had to use due to a bug…

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[1.20 regression] unreachable regression with generic Callable #21182

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[1.20 regression] unreachable regression with generic Callable #21182

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hauntsaninja commented on Apr 9, 2026

hauntsaninja commented on Apr 13, 2026

hauntsaninja commented on Apr 13, 2026

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