From 7d67aa1067896f55cbc72eb752885eac3a325e60 Mon Sep 17 00:00:00 2001
From: Ivan Levkivskyi <levkivskyi@gmail.com>
Date: Fri, 27 Feb 2026 21:36:31 +0000
Subject: [PATCH] Show an error when old and new redefinion are enabled in a
 file

---
 mypy/main.py                    |  7 +++++++
 mypy/semanal.py                 | 11 +++++++++++
 test-data/unit/check-flags.test |  6 ++++++
 3 files changed, 24 insertions(+)

diff --git a/mypy/main.py b/mypy/main.py
index bd04a427c4a23..bc20eb38ca2b7 100644
--- a/mypy/main.py
+++ b/mypy/main.py
@@ -104,6 +104,13 @@ def main(
             options,
         )
 
+    if options.allow_redefinition_new and options.allow_redefinition_old:
+        fail(
+            "--allow-redefinition-old and --allow-redefinition-new should not be used together",
+            stderr,
+            options,
+        )
+
     if options.install_types and (stdout is not sys.stdout or stderr is not sys.stderr):
         # Since --install-types performs user input, we want regular stdout and stderr.
         fail("error: --install-types not supported in this mode of running mypy", stderr, options)
diff --git a/mypy/semanal.py b/mypy/semanal.py
index e2c073fd4dea8..75994c93b1ccb 100644
--- a/mypy/semanal.py
+++ b/mypy/semanal.py
@@ -713,6 +713,17 @@ def refresh_top_level(self, file_node: MypyFile) -> None:
             n.end_line = 1
             n.end_column = 0
             self.fail("--local-partial-types must be enabled if using --allow-redefinition-new", n)
+        if self.options.allow_redefinition_new and self.options.allow_redefinition_old:
+            n = TempNode(AnyType(TypeOfAny.special_form))
+            n.line = 1
+            n.column = 0
+            n.end_line = 1
+            n.end_column = 0
+            self.fail(
+                "--allow-redefinition-old and --allow-redefinition-new"
+                " should not be used together",
+                n,
+            )
         self.recurse_into_functions = False
         self.add_implicit_module_attrs(file_node)
         for d in file_node.defs:
diff --git a/test-data/unit/check-flags.test b/test-data/unit/check-flags.test
index 5e9c7f4be8dcd..b5f8f641b539b 100644
--- a/test-data/unit/check-flags.test
+++ b/test-data/unit/check-flags.test
@@ -2737,3 +2737,9 @@ def f() -> None:
 [file mypy.ini]
 \[mypy]
 disallow_redefinition = true
+
+[case testOnlyOneRedefinitionModeAllowed]
+# flags: --allow-redefinition --allow-redefinition-new --local-partial-types
+x = 1
+[out]
+main:1: error: --allow-redefinition-old and --allow-redefinition-new should not be used together