inference results in different behavior with "is" tests

[jmesserly]

reported by @vsmenon in this comment

On a very related note, after #236, strong mode and standard Dart print different values for:

class Foo<T> {
  bool bar(x) => x is T;
}
void main() {
  Foo<int> foo = new Foo();
  print(foo.bar("hello"));
}

We used to reject this.

---

comment by me:

ah, because of inference... eeeep.

class Foo<T> {
  bool bar(x) => x is T;
}
void main() {
  Foo<int> foo = new Foo();
  var foo2 = new Foo<int>();
  print(foo.bar("hello"));
  print(foo2.bar("hello"));
}

it seems really nice if foo2 could work, even if foo doesn't.

[jmesserly]

CC @leafpetersen

I'm not sure what the right answer is. With an eye towards future Dart, does this seem like something we need to address?

[lrhn]

On Thu, May 12, 2016 at 8:06 PM, John Messerly [email protected]
wrote:

CC @leafpetersen https://github.com/leafpetersen

I'm not sure what the right answer is. With an eye towards future Dart,
does this seem like something we need to address?

We definitely need to figure out what we want. :)
With generic methods, this problem doesn't even need a class any more:

bool isit<T>(Object x) => x is T;

main() {
  print(isit<String>("ab"));
  print(isit<int>("ab"));
  print(isit("ab"));
}

/L

Lasse R.H. Nielsen - [email protected]
'Faith without judgement merely degrades the spirit divine'
Google Denmark ApS - Frederiksborggade 20B, 1 sal - 1360 København K

• Denmark - CVR nr. 28 86 69 84

[vsmenon]

Yeah, we used to reject these statically - i.e., reject is and as checks on all types where strong mode would differ (type params, non-dynamic functions, non-dynamic generics).

We could explore doing that again.

We could also allow but throw a StrongModeError at runtime if the test on a non-ground type ever returns false.

Neither are great answers, though the latter is particularly bad. :-)

[jmesserly]

@vsmenon can we close this? not sure we have any plans to address it and Dart 2 will make it obsolete

[vsmenon]

yeah, not a dev_compiler issue.