Enhancement for dartanalyzer: Emit hints for special case of function type compatibility checking

[mkustermann]

The following code does not emit a dartanalyzer error/warning/hint

import 'dart:async';

typedef void VoidReturnType(String msg);

bar(VoidReturnType function, String msg) {
  function(msg);
}

main() {
  Future fun(String m) {
    print(m);
    // The future returned here could come from other parts of the program.
    return new Future.error('unexpected error');
  }

  bar(fun, 'hello world');
}

Reading the dart specification, this seems correct behavior:

A function type (T1, ..., Tk, ...) → T is a subtype of the function type (S1, ..., Sk+j, ...) → S, if all of the following conditions are met:
Either
  S is void,
  or T ⇔ S.

But it can cause easily programming errors, where

• programmer A assumes that the caller will handle the returned Future
• programmer B who does something with the function, assumes it is void

I think this is a case where type systems usually helps users, but not in the case of the dart type system.
It would be very beneficial for our users to get at least a hint in this specific case.

More broadly speaking, it would be beneficial to get hints if a user doesn't act upon a future object it gets from somewhere (neither handles it's value nor it's errors).

[bwilkerson]

Added this to the Later milestone.
Removed Priority-Unassigned label.
Added Priority-Medium, Analyzer-Hint labels.

[kasperl]

Removed this from the Later milestone.
Added Oldschool-Milestone-Later label.

[kasperl]

Removed Oldschool-Milestone-Later label.

[srawlins]

Since the analyzer is going by the spec, and the request is for additional, false-positive-prone analysis, I think this would be a good one for the linter.