builtins.sum: Items in the iterable must support addition with int if no start value is given

stdlib/_typeshed/__init__.pyi

@@ -69,8 +69,11 @@ SupportsRichComparisonT = TypeVar("SupportsRichComparisonT", bound=SupportsRichC
 
 # Dunder protocols
 
-class SupportsAdd(Protocol):
-    def __add__(self, __x: Any) -> Any: ...
+class SupportsAdd(Protocol[_T_contra, _T_co]):
+    def __add__(self, __x: _T_contra) -> _T_co: ...
+
+class SupportsRAdd(Protocol[_T_contra, _T_co]):
+    def __radd__(self, __x: _T_contra) -> _T_co: ...
 
 class SupportsDivMod(Protocol[_T_contra, _T_co]):
     def __divmod__(self, __other: _T_contra) -> _T_co: ...

stdlib/builtins.pyi

@@ -20,6 +20,7 @@ from _typeshed import (
     SupportsKeysAndGetItem,
     SupportsLenAndGetItem,
     SupportsNext,
+    SupportsRAdd,
     SupportsRDivMod,
     SupportsRichComparison,
     SupportsRichComparisonT,
@@ -1545,8 +1546,12 @@ def sorted(
 @overload
 def sorted(__iterable: Iterable[_T], *, key: Callable[[_T], SupportsRichComparison], reverse: bool = ...) -> list[_T]: ...
 
-_SumT = TypeVar("_SumT", bound=SupportsAdd)
-_SumS = TypeVar("_SumS", bound=SupportsAdd)
+_AddableT1 = TypeVar("_AddableT1", bound=SupportsAdd[Any, Any])
+_AddableT2 = TypeVar("_AddableT2", bound=SupportsAdd[Any, Any])
+
+class _SupportsSumWithNoDefaultGiven(SupportsAdd[Any, Any], SupportsRAdd[int, Any], Protocol): ...
+
+_SupportsSumNoDefaultT = TypeVar("_SupportsSumNoDefaultT", bound=_SupportsSumWithNoDefaultGiven)
 
 # In general, the return type of `x + x` is *not* guaranteed to be the same type as x.
 # However, we can't express that in the stub for `sum()`
@@ -1561,15 +1566,15 @@ else:
     def sum(__iterable: Iterable[bool], __start: int = ...) -> int: ...  # type: ignore[misc]
 
 @overload
-def sum(__iterable: Iterable[_SumT]) -> _SumT | Literal[0]: ...
+def sum(__iterable: Iterable[_SupportsSumNoDefaultT]) -> _SupportsSumNoDefaultT | Literal[0]: ...
 
 if sys.version_info >= (3, 8):
     @overload
-    def sum(__iterable: Iterable[_SumT], start: _SumS) -> _SumT | _SumS: ...
+    def sum(__iterable: Iterable[_AddableT1], start: _AddableT2) -> _AddableT1 | _AddableT2: ...
 
 else:
     @overload
-    def sum(__iterable: Iterable[_SumT], __start: _SumS) -> _SumT | _SumS: ...
+    def sum(__iterable: Iterable[_AddableT1], __start: _AddableT2) -> _AddableT1 | _AddableT2: ...
 
 # The argument to `vars()` has to have a `__dict__` attribute, so can't be annotated with `object`
 # (A "SupportsDunderDict" protocol doesn't work)

test_cases/stdlib/builtins/test_sum.py

@@ -0,0 +1,52 @@
+# pyright: reportUnnecessaryTypeIgnoreComment=true
+
+from typing import Any, List, Union
+from typing_extensions import Literal, assert_type
+
+
+class Foo:
+    def __add__(self, other: Any) -> "Foo":
+        return Foo()
+
+
+class Bar:
+    def __radd__(self, other: Any) -> "Bar":
+        return Bar()
+
+
+class Baz:
+    def __add__(self, other: Any) -> "Baz":
+        return Baz()
+
+    def __radd__(self, other: Any) -> "Baz":
+        return Baz()
+
+
+assert_type(sum([2, 4]), int)
+assert_type(sum([3, 5], 4), int)
+
+assert_type(sum([True, False]), int)
+assert_type(sum([True, False], True), int)
+
+assert_type(sum([["foo"], ["bar"]], ["baz"]), List[str])
+
+assert_type(sum([Foo(), Foo()], Foo()), Foo)
+assert_type(sum([Baz(), Baz()]), Union[Baz, Literal[0]])
+
+# mypy and pyright infer the types differently for these, so we can't use assert_type
+# Just test that no error is emitted for any of these
+sum([("foo",), ("bar", "baz")], ())  # mypy: `tuple[str, ...]`; pyright: `tuple[()] | tuple[str] | tuple[str, str]`
+sum([5.6, 3.2])  # mypy: `float`; pyright: `float | Literal[0]`
+sum([2.5, 5.8], 5)  # mypy: `float`; pyright: `float | int`
+
+# These all fail at runtime
+sum("abcde")  # type: ignore[arg-type]
+sum([["foo"], ["bar"]])  # type: ignore[list-item]
+sum([("foo",), ("bar", "baz")])  # type: ignore[list-item]
+sum([Foo(), Foo()])  # type: ignore[list-item]
+sum([Bar(), Bar()], Bar())  # type: ignore[call-overload]
+sum([Bar(), Bar()])  # type: ignore[list-item]
+
+# TODO: these pass pyright with the current stubs, but mypy erroneously emits an error:
+# sum([3, Fraction(7, 22), complex(8, 0), 9.83])
+# sum([3, Decimal('0.98')])