dartanalyzer does not honor generic methods in strong mode without seemingly superfluous 'await'

[trinarytree]

Use dartanalyzer --strong on the following

var confuse;
Future/*<T>*/ genericMethod/*<T>*/() async => confuse as dynamic/*=T*/;

Future<int> hasNoError() async => await genericMethod();  // Apparently this is ok.
Future<int> hasError() async => genericMethod();  // "Unsound implicit
  // cast from Future<dynamic> to Future<int>"
  // Wtf?  Just because I didn't add the superfluous 'await'?

[bwilkerson]

@leafpetersen @jmesserly

[jmesserly]

Hi @trinarytree -- what is going on here is type inference couldn't figure this one out. Type inference is not perfect for Future because Future often implies a Future<T> | T union type, and we have no way of writing that in Dart. So the current inference system makes an arbitrary choice.

You can fix your code like this:

Future<int> hasError() async => genericMethod/*<int>*/();

in other words, explicitly pass the type argument, that way you don't have to rely on type inference.

There's two bugs currently assigned to me to fix inference: #25322 and #25944. I have a CL that actually does fix them, by introducing that union type as part of the type inference system. Unfortunately I haven't finished the CL yet. It needs a lot of care to make sure those union types don't escape outside the inference, as the boundary is not very clear right now in Analyzer.

I will try to get to this soon, but to keep our tracking simple, I'm going to mark this duplicate of #25322. I'll CC you over there.