From 99d4465bbb8026525ba54d82b47b02fb0fff4158 Mon Sep 17 00:00:00 2001
From: sobolevn <mail@sobolevn.me>
Date: Wed, 12 Jul 2023 18:31:42 +0300
Subject: [PATCH 1/4] Narrow `is` with final types correctly

---
 mypy/subtypes.py                    |  4 ++
 mypy/types.py                       |  9 ++++
 test-data/unit/check-narrowing.test | 76 +++++++++++++++++++++++++++++
 3 files changed, 89 insertions(+)

diff --git a/mypy/subtypes.py b/mypy/subtypes.py
index a6dc071f92b0..f0e7515e2299 100644
--- a/mypy/subtypes.py
+++ b/mypy/subtypes.py
@@ -1782,6 +1782,10 @@ def covers_at_runtime(item: Type, supertype: Type) -> bool:
     item = get_proper_type(item)
     supertype = get_proper_type(supertype)
 
+    if isinstance(item, (CallableType, TypeType)) and item.is_singleton_type():
+        if is_proper_subtype(item, supertype):
+            return True
+
     # Since runtime type checks will ignore type arguments, erase the types.
     supertype = erase_type(supertype)
     if is_proper_subtype(
diff --git a/mypy/types.py b/mypy/types.py
index ba629a3553cf..ae3f8dc49826 100644
--- a/mypy/types.py
+++ b/mypy/types.py
@@ -2085,6 +2085,9 @@ def with_unpacked_kwargs(self) -> NormalizedCallableType:
             )
         )
 
+    def is_singleton_type(self) -> bool:
+        return self.is_type_obj() and self.type_object().is_final
+
     def __hash__(self) -> int:
         # self.is_type_obj() will fail if self.fallback.type is a FakeInfo
         if isinstance(self.fallback.type, FakeInfo):
@@ -2856,6 +2859,12 @@ def __eq__(self, other: object) -> bool:
             return NotImplemented
         return self.item == other.item
 
+    def is_singleton_type(self) -> bool:
+        return (
+            (isinstance(self.item, Instance) and self.item.type.is_final)
+            or isinstance(self.item, NoneType)
+        )
+
     def serialize(self) -> JsonDict:
         return {".class": "TypeType", "item": self.item.serialize()}
 
diff --git a/test-data/unit/check-narrowing.test b/test-data/unit/check-narrowing.test
index c329ccf840a8..3b0c763824f7 100644
--- a/test-data/unit/check-narrowing.test
+++ b/test-data/unit/check-narrowing.test
@@ -1269,3 +1269,79 @@ def g() -> None:
         def foo(): ...
     foo()
 [builtins fixtures/dict.pyi]
+
+
+[case testNarrowingIsFinalType]
+from typing import Type, Union
+from typing_extensions import final
+
+@final
+class Mark: ...
+
+@final
+class Other: ...
+
+x: Union[Type[Mark], Type[Other], Type[None], int]
+
+if x is Mark:
+    reveal_type(x)  # N: Revealed type is "Type[__main__.Mark]"
+else:
+    reveal_type(x)  # N: Revealed type is "Union[Type[__main__.Other], Type[None], builtins.int]"
+
+if x is not Mark:
+    reveal_type(x)  # N: Revealed type is "Union[Type[__main__.Other], Type[None], builtins.int]"
+else:
+    reveal_type(x)  # N: Revealed type is "Type[__main__.Mark]"
+
+y: Type[Mark] = Mark
+if x is y:
+    reveal_type(x)  # N: Revealed type is "Type[__main__.Mark]"
+else:
+    reveal_type(x)  # N: Revealed type is "Union[Type[__main__.Other], Type[None], builtins.int]"
+
+if x is not y:
+    reveal_type(x)  # N: Revealed type is "Union[Type[__main__.Other], Type[None], builtins.int]"
+else:
+    reveal_type(x)  # N: Revealed type is "Type[__main__.Mark]"
+
+if x is type(None):
+    reveal_type(x)  # N: Revealed type is "Type[None]"
+else:
+    reveal_type(x)  # N: Revealed type is "Union[Type[__main__.Mark], Type[__main__.Other], builtins.int]"
+
+if x is not type(None):
+    reveal_type(x)  # N: Revealed type is "Union[Type[__main__.Mark], Type[__main__.Other], builtins.int]"
+else:
+    reveal_type(x)  # N: Revealed type is "Type[None]"
+
+z: Type[None]
+if x is z:
+    reveal_type(x)  # N: Revealed type is "Type[None]"
+else:
+    reveal_type(x)  # N: Revealed type is "Union[Type[__main__.Mark], Type[__main__.Other], builtins.int]"
+
+if x is not z:
+    reveal_type(x)  # N: Revealed type is "Union[Type[__main__.Mark], Type[__main__.Other], builtins.int]"
+else:
+    reveal_type(x)  # N: Revealed type is "Type[None]"
+[builtins fixtures/isinstancelist.pyi]
+
+
+[case testNarrowingIsRegualType]
+from typing import Type, Union
+
+class Mark: ...
+
+x: Union[Type[Mark], Type[None], int]
+
+if x is Mark:
+    reveal_type(x)  # N: Revealed type is "Union[Type[__main__.Mark], Type[None], builtins.int]"
+else:
+    reveal_type(x)  # N: Revealed type is "Union[Type[__main__.Mark], Type[None], builtins.int]"
+
+y: Type[Mark] = Mark
+if x is y:
+    reveal_type(x)  # N: Revealed type is "Union[Type[__main__.Mark], Type[None], builtins.int]"
+else:
+    reveal_type(x)  # N: Revealed type is "Union[Type[__main__.Mark], Type[None], builtins.int]"
+[builtins fixtures/isinstancelist.pyi]

From 0076ae855af496ad7dfbffdd45f4e75d2bc972f3 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Wed, 12 Jul 2023 16:22:45 +0000
Subject: [PATCH 2/4] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 mypy/types.py | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/mypy/types.py b/mypy/types.py
index ae3f8dc49826..6c99677cff5b 100644
--- a/mypy/types.py
+++ b/mypy/types.py
@@ -2860,9 +2860,8 @@ def __eq__(self, other: object) -> bool:
         return self.item == other.item
 
     def is_singleton_type(self) -> bool:
-        return (
-            (isinstance(self.item, Instance) and self.item.type.is_final)
-            or isinstance(self.item, NoneType)
+        return (isinstance(self.item, Instance) and self.item.type.is_final) or isinstance(
+            self.item, NoneType
         )
 
     def serialize(self) -> JsonDict:

From a0308bcde13e907e6dd0139f031df9956913bdd1 Mon Sep 17 00:00:00 2001
From: sobolevn <mail@sobolevn.me>
Date: Wed, 12 Jul 2023 21:07:02 +0300
Subject: [PATCH 3/4] Address primer output

---
 mypy/subtypes.py                    |  6 ++++++
 test-data/unit/check-narrowing.test | 22 ++++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/mypy/subtypes.py b/mypy/subtypes.py
index f0e7515e2299..08eebe536749 100644
--- a/mypy/subtypes.py
+++ b/mypy/subtypes.py
@@ -1782,6 +1782,12 @@ def covers_at_runtime(item: Type, supertype: Type) -> bool:
     item = get_proper_type(item)
     supertype = get_proper_type(supertype)
 
+    # Left `Any` or `Type[Any]` type will never be covered at runtime:
+    if isinstance(item, AnyType) or (
+        isinstance(item, TypeType) and isinstance(item.item, AnyType)
+    ):
+        return False
+
     if isinstance(item, (CallableType, TypeType)) and item.is_singleton_type():
         if is_proper_subtype(item, supertype):
             return True
diff --git a/test-data/unit/check-narrowing.test b/test-data/unit/check-narrowing.test
index 3b0c763824f7..87beb970574b 100644
--- a/test-data/unit/check-narrowing.test
+++ b/test-data/unit/check-narrowing.test
@@ -1327,6 +1327,28 @@ else:
 [builtins fixtures/isinstancelist.pyi]
 
 
+[case testNarrowingAnyAgainstFinalType]
+from typing import Type, Any
+from typing_extensions import final
+
+@final
+class Mark: ...
+
+x: Any
+if x is Mark:
+    reveal_type(x)  # N: Revealed type is "def () -> __main__.Mark"
+else:
+    reveal_type(x)  # N: Revealed type is "Any"
+
+y: Type[Any]
+if y is Mark:
+    reveal_type(y)  # N: Revealed type is "Type[__main__.Mark]"
+else:
+    reveal_type(y)  # N: Revealed type is "Type[Any]"
+
+[builtins fixtures/isinstancelist.pyi]
+
+
 [case testNarrowingIsRegualType]
 from typing import Type, Union
 

From 2372c12d674c5e7c8785c4c6a96f1e30f3563fc6 Mon Sep 17 00:00:00 2001
From: Shantanu <12621235+hauntsaninja@users.noreply.github.com>
Date: Thu, 17 Oct 2024 00:09:10 -0700
Subject: [PATCH 4/4] Update test-data/unit/check-narrowing.test

---
 test-data/unit/check-narrowing.test | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/test-data/unit/check-narrowing.test b/test-data/unit/check-narrowing.test
index 837e77aaef23..827a8981edfa 100644
--- a/test-data/unit/check-narrowing.test
+++ b/test-data/unit/check-narrowing.test
@@ -1452,7 +1452,7 @@ else:
 [builtins fixtures/isinstancelist.pyi]
 
 
-[case testNarrowingIsRegualType]
+[case testNarrowingIsNonFinalType]
 from typing import Type, Union
 
 class Mark: ...