From 59696d106cf4dbb4d6ab93832cf3ff66b955b2b7 Mon Sep 17 00:00:00 2001
From: STerliakov <terlya.stas@gmail.com>
Date: Mon, 12 May 2025 22:44:37 +0200
Subject: [PATCH 1/3] Admit that Final variables are never redefined

---
 mypy/checker.py                 |  4 ++++
 test-data/unit/check-final.test | 27 +++++++++++++++++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/mypy/checker.py b/mypy/checker.py
index 2d82d74cc197e..758a860abf18a 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -1557,6 +1557,10 @@ def is_var_redefined_in_outer_context(self, v: Var, after_line: int) -> bool:
         Note that this doesn't do a full CFG analysis but uses a line number based
         heuristic that isn't correct in some (rare) cases.
         """
+        if v.is_final:
+            # Final vars are definitely never reassigned.
+            return False
+
         outers = self.tscope.outer_functions()
         if not outers:
             # Top-level function -- outer context is top level, and we can't reason about
diff --git a/test-data/unit/check-final.test b/test-data/unit/check-final.test
index 4b0bab45d16ce..5189149d4743c 100644
--- a/test-data/unit/check-final.test
+++ b/test-data/unit/check-final.test
@@ -1250,3 +1250,30 @@ def check_final_init() -> None:
     new_instance = FinalInit()
     new_instance.__init__()
 [builtins fixtures/tuple.pyi]
+
+[case testNarrowingOfFinalPersistsInFunctions]
+from typing import Final, Union
+
+def _init() -> Union[int, None]:
+    return 0
+
+FOO: Final = _init()
+# For some reason the following isn't narrowed at all
+# (always stays int | None):
+# FOO: Final[Union[int, None]] = 0
+
+class Example:
+
+    if FOO is not None:
+        reveal_type(FOO)  # N: Revealed type is "builtins.int"
+
+        def fn(self) -> int:
+            return FOO
+
+if FOO is not None:
+    reveal_type(FOO)  # N: Revealed type is "builtins.int"
+
+    def func() -> int:
+        return FOO
+
+[builtins fixtures/tuple.pyi]

From 1a7599ec3ba1381c4ec585dcccfbefe23b83590e Mon Sep 17 00:00:00 2001
From: STerliakov <terlya.stas@gmail.com>
Date: Mon, 12 May 2025 23:10:35 +0200
Subject: [PATCH 2/3] Remove unneeded fixture

---
 test-data/unit/check-final.test | 2 --
 1 file changed, 2 deletions(-)

diff --git a/test-data/unit/check-final.test b/test-data/unit/check-final.test
index 5189149d4743c..d0c5b280400d0 100644
--- a/test-data/unit/check-final.test
+++ b/test-data/unit/check-final.test
@@ -1275,5 +1275,3 @@ if FOO is not None:
 
     def func() -> int:
         return FOO
-
-[builtins fixtures/tuple.pyi]

From 8af453994fccc667558644feb9c8786e366c4bcc Mon Sep 17 00:00:00 2001
From: Stanislav Terliakov <50529348+sterliakov@users.noreply.github.com>
Date: Tue, 13 May 2025 01:36:07 +0200
Subject: [PATCH 3/3] Remove comment on the known problem from the testcase

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>
---
 test-data/unit/check-final.test | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/test-data/unit/check-final.test b/test-data/unit/check-final.test
index d0c5b280400d0..d78c2a8e57f2e 100644
--- a/test-data/unit/check-final.test
+++ b/test-data/unit/check-final.test
@@ -1258,9 +1258,6 @@ def _init() -> Union[int, None]:
     return 0
 
 FOO: Final = _init()
-# For some reason the following isn't narrowed at all
-# (always stays int | None):
-# FOO: Final[Union[int, None]] = 0
 
 class Example: