From ec5eec200077b20d6c968e324f0c576bbb996e65 Mon Sep 17 00:00:00 2001
From: Pablo Galindo Salgado <pablogsal@gmail.com>
Date: Mon, 5 Oct 2026 20:07:07 +0100
Subject: [PATCH] Wait for all superclass method groups before processing
 subclasses

---
 mypy/semanal_main.py              | 10 ++++++----
 test-data/unit/check-classes.test | 12 ++++++++++++
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/mypy/semanal_main.py b/mypy/semanal_main.py
index 0f2a1e3f30ea3..876849c240f7b 100644
--- a/mypy/semanal_main.py
+++ b/mypy/semanal_main.py
@@ -26,6 +26,7 @@
 
 from __future__ import annotations
 
+from collections import Counter
 from collections.abc import Callable, Iterator
 from contextlib import nullcontext
 from itertools import groupby
@@ -222,10 +223,9 @@ def order_by_subclassing(targets: list[FullTargetInfo]) -> Iterator[FullTargetIn
     that are already almost correctly ordered.
     """
 
-    # First, group the targets by their TypeInfo (since targets are sorted by line,
-    # we know that each TypeInfo will appear as group key only once).
+    # A nested class can split its enclosing class's methods into multiple groups.
     grouped = [(k, list(g)) for k, g in groupby(targets, key=lambda x: x[3])]
-    remaining_infos = {info for info, _ in grouped if info is not None}
+    remaining_infos = Counter(info for info, _ in grouped if info is not None)
 
     next_group = 0
     while grouped:
@@ -245,7 +245,9 @@ def order_by_subclassing(targets: list[FullTargetInfo]) -> Iterator[FullTargetIn
             continue
         yield from group
         grouped.pop(next_group)
-        remaining_infos.discard(next_info)
+        remaining_infos[next_info] -= 1
+        if not remaining_infos[next_info]:
+            del remaining_infos[next_info]
         # Each time after processing a method group we should retry from start,
         # since there may be some groups that are not blocked on parents anymore.
         next_group = 0
diff --git a/test-data/unit/check-classes.test b/test-data/unit/check-classes.test
index fa742f3571f82..3ee2b02726936 100644
--- a/test-data/unit/check-classes.test
+++ b/test-data/unit/check-classes.test
@@ -7322,6 +7322,18 @@ class E(C):
 
 [targets __main__, __main__, __main__.C.__init__, __main__.D.g, __main__.D.f, __main__.E.g, __main__.E.f]
 
+[case testAttributeDefOrderNestedClass]
+class E(D):
+    def e(self) -> None:
+        self.x = ''  # E: Incompatible types in assignment (expression has type "str", variable has type "int")
+
+class D:
+    def d(self) -> None: pass
+    class N:
+        def n(self) -> None: pass
+    def d2(self) -> None:
+        self.x = 0
+
 [case testNewReturnType1]
 class A:
     def __new__(cls) -> B: