builtins.sum: Items in the iterable must support addition with int if no start value is given (#8000)

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')])