Nullable future generated as return type for async functions that return a nullable value

[adwiv]

Issue description

The return type generated by quickfix in vscode, studio or dartpad for the following async function is Future<String?>?. In my opinion, it should be Future<String?> instead, as an async function would always return a Future and never null.

returnStringOrNull() async 
{
  return Random().nextBool() ? "OK" : null;
}

Dart SDK version: 3.2.6 (stable) (Wed Jan 24 13:41:58 2024 +0000) on "macos_x64"

Steps to replicate

Here is a dartpad showing the same issue - https://dartpad.dev/?id=0e8505321de7157abe72675f4e1ab073

Right click on returnStringOrNull and then select Add return type. The resulting code is

Future<String?>? returnStringOrNull() async 
{
  return Random().nextBool() ? "OK" : null;
}