From 4c8b224851bd2109915c308ab8653b58a27fdf2b Mon Sep 17 00:00:00 2001
From: Khanh Le <nkgotcode@users.noreply.github.com>
Date: Wed, 17 Jun 2026 12:25:33 -0500
Subject: [PATCH 1/3] Fix TypeIs narrowing before isinstance

---
 mypy/checker.py                  | 18 +++++++++++++++---
 test-data/unit/check-typeis.test | 19 +++++++++++++++++++
 2 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/mypy/checker.py b/mypy/checker.py
index 33705c98e10c3..8ede926a6d23b 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -6591,11 +6591,23 @@ def find_isinstance_check_helper(
                 if len(node.args) != 2:  # the error will be reported elsewhere
                     return {}, {}
                 if literal(expr) == LITERAL_TYPE:
+                    original_type = self.lookup_type(expr)
+                    current_type = self.expr_checker.narrow_type_from_binder(
+                        expr, original_type
+                    )
+                    yes_type, no_type = self.conditional_types_with_intersection(
+                        current_type, self.get_isinstance_type(node.args[1]), expr
+                    )
+                    if (
+                        self.binder.get(expr) is not None
+                        and yes_type is not None
+                        and is_subtype(current_type, yes_type, ignore_promotions=True)
+                    ):
+                        yes_type = current_type
                     return conditional_types_to_typemaps(
                         expr,
-                        *self.conditional_types_with_intersection(
-                            self.lookup_type(expr), self.get_isinstance_type(node.args[1]), expr
-                        ),
+                        yes_type,
+                        no_type,
                     )
             elif refers_to_fullname(node.callee, "builtins.issubclass"):
                 if len(node.args) != 2:  # the error will be reported elsewhere
diff --git a/test-data/unit/check-typeis.test b/test-data/unit/check-typeis.test
index 65ee837452f5b..6b63f9aa080cf 100644
--- a/test-data/unit/check-typeis.test
+++ b/test-data/unit/check-typeis.test
@@ -26,6 +26,25 @@ def main(a: Union[Point, Line, int]) -> None:
 
 [builtins fixtures/tuple.pyi]
 
+[case testTypeIsAndIsinstanceWithGenericAlias]
+from typing import Any, Generic, TypeVar
+from typing_extensions import TypeAlias, TypeIs
+
+T = TypeVar("T")
+class Slice(Generic[T]): pass
+
+SliceInt: TypeAlias = Slice[int | None]
+SliceStr: TypeAlias = Slice[str | None]
+
+def is_slice_int(obj: Any) -> TypeIs[SliceInt]: pass
+
+def main(obj: SliceInt | SliceStr) -> None:
+    if is_slice_int(obj):
+        pass
+    elif isinstance(obj, Slice):
+        reveal_type(obj)  # N: Revealed type is "__main__.Slice[builtins.str | None]"
+[builtins fixtures/isinstance.pyi]
+
 [case testTypeIsTypeArgsNone]
 from typing_extensions import TypeIs
 def foo(a: object) -> TypeIs:  # E: TypeIs must have exactly one type argument

From 0af3bc4130431b23df16fa8f01610a4d86a5c26f Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Wed, 17 Jun 2026 17:35:32 +0000
Subject: [PATCH 2/3] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 mypy/checker.py | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/mypy/checker.py b/mypy/checker.py
index 8ede926a6d23b..c2409661a6e2f 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -6592,9 +6592,7 @@ def find_isinstance_check_helper(
                     return {}, {}
                 if literal(expr) == LITERAL_TYPE:
                     original_type = self.lookup_type(expr)
-                    current_type = self.expr_checker.narrow_type_from_binder(
-                        expr, original_type
-                    )
+                    current_type = self.expr_checker.narrow_type_from_binder(expr, original_type)
                     yes_type, no_type = self.conditional_types_with_intersection(
                         current_type, self.get_isinstance_type(node.args[1]), expr
                     )
@@ -6604,11 +6602,7 @@ def find_isinstance_check_helper(
                         and is_subtype(current_type, yes_type, ignore_promotions=True)
                     ):
                         yes_type = current_type
-                    return conditional_types_to_typemaps(
-                        expr,
-                        yes_type,
-                        no_type,
-                    )
+                    return conditional_types_to_typemaps(expr, yes_type, no_type)
             elif refers_to_fullname(node.callee, "builtins.issubclass"):
                 if len(node.args) != 2:  # the error will be reported elsewhere
                     return {}, {}

From 8fc57a23b212c40173e163583cd0e20c08e12673 Mon Sep 17 00:00:00 2001
From: Khanh Le <nkgotcode@users.noreply.github.com>
Date: Wed, 17 Jun 2026 12:44:25 -0500
Subject: [PATCH 3/3] Fix isinstance narrowing from Any

---
 mypy/checker.py                      | 1 +
 test-data/unit/check-isinstance.test | 9 +++++++++
 2 files changed, 10 insertions(+)

diff --git a/mypy/checker.py b/mypy/checker.py
index c2409661a6e2f..4c643bb4317b3 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -6599,6 +6599,7 @@ def find_isinstance_check_helper(
                     if (
                         self.binder.get(expr) is not None
                         and yes_type is not None
+                        and not isinstance(get_proper_type(current_type), AnyType)
                         and is_subtype(current_type, yes_type, ignore_promotions=True)
                     ):
                         yes_type = current_type
diff --git a/test-data/unit/check-isinstance.test b/test-data/unit/check-isinstance.test
index acd81839fcdc7..b63b5f6c8a6f4 100644
--- a/test-data/unit/check-isinstance.test
+++ b/test-data/unit/check-isinstance.test
@@ -3248,6 +3248,15 @@ def foo(x: object, t: type[Any]):
         reveal_type(x)  # N: Revealed type is "Any"
 [builtins fixtures/isinstance.pyi]
 
+[case testAssertIsinstanceAny]
+from typing import Any
+
+def f(x: Any) -> str:
+    assert isinstance(x, str)
+    reveal_type(x)  # N: Revealed type is "builtins.str"
+    return x
+[builtins fixtures/isinstance.pyi]
+
 [case testIsInstanceObject]
 # flags: --strict-equality --warn-unreachable
 from typing import Any