Front end infers dynamic for ".then" invocations

[stereotype441]

Consider the following code:

void test(Future<int> f) {
  var t1 = f.then((int x) {});
  var t2 = f.then((int x) { return; });
  var t3 = f.then((int x) { return null; });
  var t4 = f.then((int x) => null);
  var t5 = f.then((int x) { return x.toString(); });
  var t6 = f.then((int x) => x.toString());
  var t7 = f.then((x) {});
  var t8 = f.then((x) { return; });
  var t9 = f.then((x) { return null; });
  var t10 = f.then((x) => null);
  var t11 = f.then((x) { return x.toString(); });
  var t12 = f.then((x) => x.toString());
}

The analyzer infers t5, t6, t11, and t12 to have type Future<String>, and the others to have type Future<Null>. The front end infers the type of t1 through t12 to be dynamic.

This has user-visible consequences. For example, the analyzer considers this to be an error, and the front end doesn't:

void test(Future<int> f) async {
  var t = f.then((int x) { return x.toString(); });
  print(t.foo);
}

I believe the analyzer behavior is correct. @leafpetersen can you confirm?

[leafpetersen]

I suspect that all of the .toString examples are instances of this bug: #32414 .

For the remainder: Future<dynamic> is always wrong. For t1, t2, t7, and t8 the analyzer is still implementing legacy behavior: we should infer Future<void> there, and I will be working on that change as soon as a fix for return warnings has rolled internally (unblocking the fixes for this).

For the rest, Future<Null> is correct.

[leafpetersen]

#32843 is the issue for void inference.

[stereotype441]

I suspect that all of the .toString examples are instances of this bug: #32414 .

@leafpetersen in that case I would have expected the front end to infer a type of Future<dynamic>. But it's inferring a type of dynamic.

[leafpetersen]

sorry, I didn't read closely enough. I can't reproduce this though: the code below, run with dart --preview-dart-2 from fairly recent bleeding edge, has the expected errors (I think it stops reporting errors after some limit, but if you delete the first 5, you get the next 5 as expected). Am I still missing something?

import 'dart:async';

void test(Future<int> f) {
  int x;
  var t1 = f.then((int x) {});
  x = t1;
  t1.foo;
  var t2 = f.then((int x) { return; });
  x = t2;
  t2.foo;
  var t3 = f.then((int x) { return null; });
  x = t3;
  t3.foo;
  var t4 = f.then((int x) => null);
  x = t4;
  t4.foo;
  var t5 = f.then((int x) { return x.toString(); });
  x = t5;
  t5.foo;
  var t6 = f.then((int x) => x.toString());
  x = t6;
  t6.foo;
  var t7 = f.then((x) {});
  x = t7;
  t7.foo;
  var t8 = f.then((x) { return; });
  x = t8;
  t8.foo;
  var t9 = f.then((x) { return null; });
  x = t9;
  t9.foo;
  var t10 = f.then((x) => null);
  x = t10;
  t10.foo;
  var t11 = f.then((x) { return x.toString(); });
  x = t11;
  t11.foo;
  var t12 = f.then((x) => x.toString());
  x = t12;
  t12.foo;
}

void main() {
  test(new Future.value(3));
}

[stereotype441]

@leafpetersen Oops, you are right. I forgot to include import 'dart:async'; in my test script (and failed to notice the error message the front end issued).

When I include the correct import, I get the same results as you. Since the remaining problems are already covered by open bugs, I'm closing this bug.