From 0c41e090472ecb0be259f64e1f17d36364395ff4 Mon Sep 17 00:00:00 2001
From: zhengguangzhuo <100548634+zhengguangzhuo@users.noreply.github.com>
Date: Thu, 22 Oct 2026 17:21:56 +0800
Subject: [PATCH] Fix narrowing after always-true blocks with
 allow-redefinition

---
 mypy/binder.py                      |  2 ++
 mypy/checker.py                     |  3 ++-
 test-data/unit/check-redefine2.test | 38 +++++++++++++++++++++++++++++
 3 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/mypy/binder.py b/mypy/binder.py
index 9984781733cd4..cd6d9ae3ff89e 100644
--- a/mypy/binder.py
+++ b/mypy/binder.py
@@ -398,6 +398,8 @@ def update_from_options(self, frames: list[Frame]) -> bool:
             if (
                 current_value is None
                 or not is_same_type(type, current_value.type)
+                # 新语义下，即使类型语义等价，也要保留合并得到的窄化表示。
+                or (not old_semantics and type != current_value.type)
                 # Manually carry over any narrowing from hasattr() from inner frames. This is
                 # a bit ad-hoc, but our handling of hasattr() is on best effort basis anyway.
                 or isinstance(p_type := get_proper_type(type), Instance)
diff --git a/mypy/checker.py b/mypy/checker.py
index fd4486218dc11..1f40f4fc86b28 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -5477,7 +5477,8 @@ def visit_if_stmt(self, s: IfStmt) -> None:
                 s.unreachable_else = is_unreachable_map(else_map)
 
                 # XXX Issue a warning if condition is always False?
-                with self.binder.frame_context(can_skip=True, fall_through=2):
+                # else 分支不可达时，不能把进入 if 前的状态作为可跳过路径合并。
+                with self.binder.frame_context(can_skip=not s.unreachable_else, fall_through=2):
                     self.push_type_map(if_map, from_assignment=False)
                     self.accept(b)
 
diff --git a/test-data/unit/check-redefine2.test b/test-data/unit/check-redefine2.test
index b5a8aeac0e3da..12a352c4c0c4f 100644
--- a/test-data/unit/check-redefine2.test
+++ b/test-data/unit/check-redefine2.test
@@ -1297,6 +1297,44 @@ def foo() -> None:
         if bool():
             name = "a"
 
+[case testNarrowingSurvivesAlwaysTrueBlock]
+# flags: --allow-redefinition
+class Base: ...
+class Sub(Base): ...
+
+def f(x: Sub | Base) -> None:
+    if True:
+        assert not isinstance(x, Sub)
+        reveal_type(x)  # N: Revealed type is "__main__.Base"
+    reveal_type(x)  # N: Revealed type is "__main__.Base"
+[builtins fixtures/isinstance.pyi]
+
+[case testNarrowingSurvivesAlwaysTrueBlockDefault]
+class Base: ...
+class Sub(Base): ...
+
+def f(x: Sub | Base) -> None:
+    if True:
+        assert not isinstance(x, Sub)
+        reveal_type(x)  # N: Revealed type is "__main__.Base"
+    reveal_type(x)  # N: Revealed type is "__main__.Base"
+[builtins fixtures/isinstance.pyi]
+
+[case testNarrowingSurvivesTypeCheckingBlock]
+# flags: --allow-redefinition
+from typing import TYPE_CHECKING
+
+class Base: ...
+class Sub(Base): ...
+
+def f(x: Sub | Base) -> None:
+    if TYPE_CHECKING:
+        assert not isinstance(x, Sub)
+        reveal_type(x)  # N: Revealed type is "__main__.Base"
+    reveal_type(x)  # N: Revealed type is "__main__.Base"
+[builtins fixtures/isinstance.pyi]
+[typing fixtures/typing-full.pyi]
+
 [case testNewRedefinePartialTypeForUnderscore]
 # flags: --allow-redefinition