From 5a08ce14e2f90c5ab5e636c2e4294e6ddf104ebc Mon Sep 17 00:00:00 2001
From: Brian Schubert <brianm.schubert@gmail.com>
Date: Wed, 25 Sep 2024 10:08:11 -0400
Subject: [PATCH 1/3] Signal unreachability when narrowing with
 Type{Guard,Is}[Never]

---
 mypy/checker.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mypy/checker.py b/mypy/checker.py
index 9c4f4ce88690..c8416544e11b 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -5889,9 +5889,13 @@ def find_isinstance_check_helper(
                         # Also note that a care must be taken to unwrap this back at read places
                         # where we use this to narrow down declared type.
                         if node.callee.type_guard is not None:
+                            if isinstance(get_proper_type(node.callee.type_guard), UninhabitedType):
+                                return None, {}
                             return {expr: TypeGuardedType(node.callee.type_guard)}, {}
                         else:
                             assert node.callee.type_is is not None
+                            if isinstance(get_proper_type(node.callee.type_is), UninhabitedType):
+                                return None, {}
                             return conditional_types_to_typemaps(
                                 expr,
                                 *self.conditional_types_with_intersection(

From 29e25c603031ce953597edd920a25856ab00ffea Mon Sep 17 00:00:00 2001
From: Brian Schubert <brianm.schubert@gmail.com>
Date: Wed, 25 Sep 2024 11:05:29 -0400
Subject: [PATCH 2/3] Add tests

---
 test-data/unit/check-unreachable-code.test | 38 ++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/test-data/unit/check-unreachable-code.test b/test-data/unit/check-unreachable-code.test
index cbad1bd5449e..794e6c67e3f4 100644
--- a/test-data/unit/check-unreachable-code.test
+++ b/test-data/unit/check-unreachable-code.test
@@ -1507,3 +1507,41 @@ x = 0  # not unreachable
 
 f2: Callable[[], NoReturn] = lambda: foo()
 x = 0  # not unreachable
+
+[case testUnreachableTypeGuardNever]
+# flags: --warn-unreachable
+from typing_extensions import Never, TypeGuard
+
+def guard(x: object) -> TypeGuard[Never]:
+    pass
+
+a: object
+assert not guard(a)
+
+if guard(a):
+    reveal_type(a)  # E: Statement is unreachable
+else:
+    reveal_type(a)  # N: Revealed type is "builtins.object"
+
+assert guard(a)
+b = 0  # E: Statement is unreachable
+[builtins fixtures/tuple.pyi]
+
+[case testUnreachableTypeIsNever]
+# flags: --warn-unreachable
+from typing_extensions import Never, TypeIs
+
+def guard(x: object) -> TypeIs[Never]:
+    pass
+
+a: object
+assert not guard(a)
+
+if guard(a):
+    reveal_type(a)  # E: Statement is unreachable
+else:
+    reveal_type(a)  # N: Revealed type is "builtins.object"
+
+assert guard(a)
+b = 0  # E: Statement is unreachable
+[builtins fixtures/tuple.pyi]

From 9e8008a98e1dad0f2be298eff8c37a96fed3723e Mon Sep 17 00:00:00 2001
From: Brian Schubert <brianm.schubert@gmail.com>
Date: Wed, 25 Sep 2024 11:45:33 -0400
Subject: [PATCH 3/3] Black format

---
 mypy/checker.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mypy/checker.py b/mypy/checker.py
index c8416544e11b..db172823b403 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -5889,7 +5889,9 @@ def find_isinstance_check_helper(
                         # Also note that a care must be taken to unwrap this back at read places
                         # where we use this to narrow down declared type.
                         if node.callee.type_guard is not None:
-                            if isinstance(get_proper_type(node.callee.type_guard), UninhabitedType):
+                            if isinstance(
+                                get_proper_type(node.callee.type_guard), UninhabitedType
+                            ):
                                 return None, {}
                             return {expr: TypeGuardedType(node.callee.type_guard)}, {}
                         else: