From 652bee088fcbda2d28a018d572b5be23b9f67abe Mon Sep 17 00:00:00 2001
From: Brian Schubert <brianm.schubert@gmail.com>
Date: Sun, 6 Oct 2024 23:21:23 -0400
Subject: [PATCH 1/4] Emit error on 'raise NotImplemented'

---
 mypy/checker.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mypy/checker.py b/mypy/checker.py
index 4bbd49cd71984..a5a4ab727995c 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -4746,6 +4746,14 @@ def type_check_raise(self, e: Expression, s: RaiseStmt, optional: bool = False)
             # https://github.com/python/mypy/issues/11089
             self.expr_checker.check_call(typ, [], [], e)
 
+        if isinstance(typ, Instance) and typ.type.fullname == "builtins._NotImplementedType":
+            self.fail(
+                message_registry.INVALID_EXCEPTION.with_additional_msg(
+                    '; did you mean "NotImplementedError"?'
+                ),
+                s,
+            )
+
     def visit_try_stmt(self, s: TryStmt) -> None:
         """Type check a try statement."""
         # Our enclosing frame will get the result if the try/except falls through.

From 7c4a67e0e5bfbcc2ee910815c11189e9a9b5ee85 Mon Sep 17 00:00:00 2001
From: Brian Schubert <brianm.schubert@gmail.com>
Date: Mon, 7 Oct 2024 10:01:45 -0400
Subject: [PATCH 2/4] Add test case for raising NotImplemented

---
 test-data/unit/check-statements.test       |  7 +++++++
 test-data/unit/fixtures/notimplemented.pyi | 10 +++++++---
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/test-data/unit/check-statements.test b/test-data/unit/check-statements.test
index d1464423e90f0..44880cf352043 100644
--- a/test-data/unit/check-statements.test
+++ b/test-data/unit/check-statements.test
@@ -519,6 +519,13 @@ if object():
     raise BaseException from f # E: Exception must be derived from BaseException
 [builtins fixtures/exception.pyi]
 
+[case testRaiseNotImplementedFails]
+if object():
+    raise NotImplemented    # E: Exception must be derived from BaseException; did you mean "NotImplementedError"?
+if object():
+    raise NotImplemented()  # E: NotImplemented? not callable
+[builtins fixtures/notimplemented.pyi]
+
 [case testTryFinallyStatement]
 import typing
 try:
diff --git a/test-data/unit/fixtures/notimplemented.pyi b/test-data/unit/fixtures/notimplemented.pyi
index 2ca376ea07605..c13cfe0114274 100644
--- a/test-data/unit/fixtures/notimplemented.pyi
+++ b/test-data/unit/fixtures/notimplemented.pyi
@@ -1,6 +1,5 @@
 # builtins stub used in NotImplemented related cases.
-from typing import Any, cast
-
+from typing import Any
 
 class object:
     def __init__(self) -> None: pass
@@ -10,5 +9,10 @@ class function: pass
 class bool: pass
 class int: pass
 class str: pass
-NotImplemented = cast(Any, None)
 class dict: pass
+
+class _NotImplementedType(Any):
+    __call__: NotImplemented  # type: ignore
+NotImplemented = _NotImplementedType()
+
+class BaseException: pass

From a46afa702c28426ff59740ebc1965d02ca2138b9 Mon Sep 17 00:00:00 2001
From: Brian Schubert <brianm.schubert@gmail.com>
Date: Mon, 7 Oct 2024 10:27:56 -0400
Subject: [PATCH 3/4] Fix test after syncing NotImplemented fixture definition
 with typeshed

Test case testReturnAnyForNotImplementedInNormalMethods become silently
invalid after typeshed changes it defintiion of NotImplemented from
Any to a subclass to Any.

We can store the old behavior by special casing NotImplemented in
check_return_stmt.
---
 mypy/checker.py | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mypy/checker.py b/mypy/checker.py
index a5a4ab727995c..9021039e6e19b 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -4581,6 +4581,13 @@ def check_return_stmt(self, s: ReturnStmt) -> None:
                         s.expr, return_type, allow_none_return=allow_none_func_call
                     )
                 )
+                # Treat NotImplemented as having type Any, consistent with its
+                # definition in typeshed prior to python/typeshed#4222.
+                if (
+                    isinstance(typ, Instance)
+                    and typ.type.fullname == "builtins._NotImplementedType"
+                ):
+                    typ = AnyType(TypeOfAny.special_form)
 
                 if defn.is_async_generator:
                     self.fail(message_registry.RETURN_IN_ASYNC_GENERATOR, s)

From bf3a8a902f19907f4657178bd37ac3a2772601cb Mon Sep 17 00:00:00 2001
From: Brian Schubert <brianm.schubert@gmail.com>
Date: Tue, 8 Oct 2024 13:53:56 -0400
Subject: [PATCH 4/4] Tweak NotImplemented definition in
 fixtures/notimplemented.pyi

---
 test-data/unit/fixtures/notimplemented.pyi | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/test-data/unit/fixtures/notimplemented.pyi b/test-data/unit/fixtures/notimplemented.pyi
index c13cfe0114274..92edf84a7fd11 100644
--- a/test-data/unit/fixtures/notimplemented.pyi
+++ b/test-data/unit/fixtures/notimplemented.pyi
@@ -13,6 +13,6 @@ class dict: pass
 
 class _NotImplementedType(Any):
     __call__: NotImplemented  # type: ignore
-NotImplemented = _NotImplementedType()
+NotImplemented: _NotImplementedType
 
 class BaseException: pass