Fix asyncio.gather regression (#8271)

Adding the empty-tuple overload caused major problems for pyright, and that overload only deals with an unlikely edge case anyway. Get rid of it, and replace the fallback overload with a more general overload.

Fixes #8270.

test_cases/stdlib/asyncio/test_gather.py

@@ -0,0 +1,32 @@
+import asyncio
+from typing import Any, Awaitable, List, Tuple, Union
+from typing_extensions import assert_type
+
+
+async def coro1() -> int:
+    return 42
+
+
+async def coro2() -> str:
+    return "spam"
+
+
+async def test_gather(awaitable1: Awaitable[int], awaitable2: Awaitable[str]) -> None:
+    a = await asyncio.gather(awaitable1)
+    assert_type(a, Tuple[int])
+
+    b = await asyncio.gather(awaitable1, awaitable2, return_exceptions=True)
+    assert_type(b, Tuple[Union[int, BaseException], Union[str, BaseException]])
+
+    c = await asyncio.gather(awaitable1, awaitable2, awaitable1, awaitable1, awaitable1, awaitable1)
+    assert_type(c, List[Any])
+
+    awaitables_list: List[Awaitable[int]] = [awaitable1]
+    d = await asyncio.gather(*awaitables_list)
+    assert_type(d, List[Any])
+
+    e = await asyncio.gather()
+    assert_type(e, List[Any])
+
+
+asyncio.run(test_gather(coro1(), coro2()))