From 2cf2fc14672430c82763b3eb934c10909cae6ecf Mon Sep 17 00:00:00 2001
From: TheMuffinMan1320 <76186189+TheMuffinMan1320@users.noreply.github.com>
Date: Thu, 13 Aug 2026 14:48:08 -0500
Subject: [PATCH] Fix crash when singledispatch is shadowed by a user function
 (#20339)

The functools.singledispatch plugin hook is matched purely by fully
qualified name. If a user's own module happens to produce a function
whose fullname is exactly "functools.singledispatch" (e.g. a
top-level module literally named functools.py defining its own
singledispatch function), the hook fires even though the return type
is not an Instance of functools._SingleDispatchCallable, tripping an
assert and crashing mypy. Bail out gracefully instead.
---
 mypy/plugins/singledispatch.py           |  3 ++-
 test-data/unit/check-singledispatch.test | 12 ++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/mypy/plugins/singledispatch.py b/mypy/plugins/singledispatch.py
index 9a5576c17e82c..7fb01985f8e4a 100644
--- a/mypy/plugins/singledispatch.py
+++ b/mypy/plugins/singledispatch.py
@@ -106,7 +106,8 @@ def create_singledispatch_function_callback(ctx: FunctionContext) -> Type:
         # singledispatch returns an instance of functools._SingleDispatchCallable according to
         # typeshed
         singledispatch_obj = get_proper_type(ctx.default_return_type)
-        assert isinstance(singledispatch_obj, Instance)
+        if not isinstance(singledispatch_obj, Instance):
+            return ctx.default_return_type
         singledispatch_obj.args += (func_type,)
 
     return ctx.default_return_type
diff --git a/test-data/unit/check-singledispatch.test b/test-data/unit/check-singledispatch.test
index 36c99b288bbe8..eb172acba3673 100644
--- a/test-data/unit/check-singledispatch.test
+++ b/test-data/unit/check-singledispatch.test
@@ -307,3 +307,15 @@ def default(val) -> int:
     return 3
 
 [builtins fixtures/args.pyi]
+
+[case testDontCrashWhenSingledispatchIsShadowedByUserFunction]
+# cmd: mypy -m functools
+[file functools.py]
+def singledispatch(func):
+    pass
+
+@singledispatch
+def f(fn):
+    pass
+[out]
+[builtins fixtures/args.pyi]