Refactor round and add tests (#9151)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

test_cases/stdlib/builtins/check_round.py

@@ -0,0 +1,70 @@
+from __future__ import annotations
+
+from typing import overload
+from typing_extensions import assert_type
+
+
+class CustomIndex:
+    def __index__(self) -> int:
+        return 1
+
+
+# float:
+
+assert_type(round(5.5), int)
+assert_type(round(5.5, None), int)
+assert_type(round(5.5, 0), float)
+assert_type(round(5.5, 1), float)
+assert_type(round(5.5, 5), float)
+assert_type(round(5.5, CustomIndex()), float)
+
+# int:
+
+assert_type(round(1), int)
+assert_type(round(1, 1), int)
+assert_type(round(1, None), int)
+assert_type(round(1, CustomIndex()), int)
+
+# Protocols:
+
+
+class WithCustomRound1:
+    def __round__(self) -> str:
+        return "a"
+
+
+assert_type(round(WithCustomRound1()), str)
+assert_type(round(WithCustomRound1(), None), str)
+# Errors:
+round(WithCustomRound1(), 1)  # type: ignore
+round(WithCustomRound1(), CustomIndex())  # type: ignore
+
+
+class WithCustomRound2:
+    def __round__(self, digits: int) -> str:
+        return "a"
+
+
+assert_type(round(WithCustomRound2(), 1), str)
+assert_type(round(WithCustomRound2(), CustomIndex()), str)
+# Errors:
+round(WithCustomRound2(), None)  # type: ignore
+round(WithCustomRound2())  # type: ignore
+
+
+class WithOverloadedRound:
+    @overload
+    def __round__(self, ndigits: None = ...) -> str:
+        ...
+
+    @overload
+    def __round__(self, ndigits: int) -> bytes:
+        ...
+
+    def __round__(self, ndigits: int | None = None) -> str | bytes:
+        return b"" if ndigits is None else ""
+
+
+assert_type(round(WithOverloadedRound()), str)
+assert_type(round(WithOverloadedRound(), None), str)
+assert_type(round(WithOverloadedRound(), 1), bytes)