From b5ad828e7e451b67971ea491096585efb6c4f64a Mon Sep 17 00:00:00 2001
From: KotlinIsland <65446343+kotlinisland@users.noreply.github.com>
Date: Sun, 21 Sep 2025 20:29:15 +1000
Subject: [PATCH 1/2] fix `list.__add__` to use the expected type

---
 .../@tests/test_cases/builtins/check_list.py  | 21 +++++++++++++++++++
 stdlib/builtins.pyi                           |  5 +++--
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/stdlib/@tests/test_cases/builtins/check_list.py b/stdlib/@tests/test_cases/builtins/check_list.py
index 4113f5c66182..07e443c35c12 100644
--- a/stdlib/@tests/test_cases/builtins/check_list.py
+++ b/stdlib/@tests/test_cases/builtins/check_list.py
@@ -19,3 +19,24 @@ def asd(self) -> int:
 assert_type(combined, List[Union[Foo, Bar]])
 for item in combined:
     assert_type(item.asd(), int)
+
+# ignoring this so we can test mypy and pyright separately
+# pyright: reportUnnecessaryTypeIgnoreComment=false
+
+# defining separately so that the value is not inferred at the usage
+l_int = [1, 2]
+l_str = ["a", "b"]
+combined1 = l_int + l_str
+assert_type(combined1, List[Union[int, str]])
+
+combined2: list[Union[int, str]]
+# mypy doesn't support this case
+combined2 = l_int + l_str  # type: ignore[operator]
+assert_type(combined2, List[Union[int, str]])
+
+combined2 = l_int + combined1
+assert_type(combined2, List[Union[int, str]])
+
+# mypy doesn't support this case
+combined3: List[object] = l_int + l_str  # type: ignore[operator]
+assert_type(combined3, List[object])
diff --git a/stdlib/builtins.pyi b/stdlib/builtins.pyi
index 304e94eafa61..8afc8f432e7e 100644
--- a/stdlib/builtins.pyi
+++ b/stdlib/builtins.pyi
@@ -1137,9 +1137,10 @@ class list(MutableSequence[_T]):
     def __delitem__(self, key: SupportsIndex | slice, /) -> None: ...
     # Overloading looks unnecessary, but is needed to work around complex mypy problems
     @overload
-    def __add__(self, value: list[_T], /) -> list[_T]: ...
+    # # `__add__` returns a new object, so we capture the expected result type with a type variable
+    def __add__(self, value: list[_T], /) -> list[_T1 | _T]: ...
     @overload
-    def __add__(self, value: list[_S], /) -> list[_S | _T]: ...
+    def __add__(self, value: list[_S], /) -> list[_T1 | _T | _S]: ...
     def __iadd__(self, value: Iterable[_T], /) -> Self: ...  # type: ignore[misc]
     def __mul__(self, value: SupportsIndex, /) -> list[_T]: ...
     def __rmul__(self, value: SupportsIndex, /) -> list[_T]: ...

From d8db8975102cf35dfcc4e84320f0f6045a6ecddf Mon Sep 17 00:00:00 2001
From: KotlinIsland <65446343+kotlinisland@users.noreply.github.com>
Date: Wed, 1 Oct 2025 11:25:25 +1000
Subject: [PATCH 2/2] better name for the typevar

---
 stdlib/builtins.pyi | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/stdlib/builtins.pyi b/stdlib/builtins.pyi
index 8afc8f432e7e..b288f45fd9e5 100644
--- a/stdlib/builtins.pyi
+++ b/stdlib/builtins.pyi
@@ -90,6 +90,7 @@ _T2 = TypeVar("_T2")
 _T3 = TypeVar("_T3")
 _T4 = TypeVar("_T4")
 _T5 = TypeVar("_T5")
+_Expected = TypeVar("_Expected")
 _SupportsNextT_co = TypeVar("_SupportsNextT_co", bound=SupportsNext[Any], covariant=True)
 _SupportsAnextT_co = TypeVar("_SupportsAnextT_co", bound=SupportsAnext[Any], covariant=True)
 _AwaitableT = TypeVar("_AwaitableT", bound=Awaitable[Any])
@@ -1138,9 +1139,9 @@ class list(MutableSequence[_T]):
     # Overloading looks unnecessary, but is needed to work around complex mypy problems
     @overload
     # # `__add__` returns a new object, so we capture the expected result type with a type variable
-    def __add__(self, value: list[_T], /) -> list[_T1 | _T]: ...
+    def __add__(self, value: list[_T], /) -> list[_Expected | _T]: ...
     @overload
-    def __add__(self, value: list[_S], /) -> list[_T1 | _T | _S]: ...
+    def __add__(self, value: list[_S], /) -> list[_Expected | _T | _S]: ...
     def __iadd__(self, value: Iterable[_T], /) -> Self: ...  # type: ignore[misc]
     def __mul__(self, value: SupportsIndex, /) -> list[_T]: ...
     def __rmul__(self, value: SupportsIndex, /) -> list[_T]: ...