Issues blocking Flutter's use of "implicit-casts: false".

[Hixie]

We get 1668 errors when using implicit-casts: false today.

I looked at some of them and found the following broad issues:

1. We often have variables of type dynamic, which we then pass to methods that expect particular types. I think we want this to do a type check at runtime then cast automatically. The point of dynamic is that it opts you out of static typing.

2. We have methods that take type arguments, do a search of a tree or Map or whatnot, then return an instance of the given type. The call sites assume the type is correct. Today, they take their type as a regular argument, and the return type is therefore not as tight as it could be. We would switch to generic type arguments to pass the type (allowing us to set the return type correctly), but the VM doesn't reify the types and we therefore couldn't actually do the search.

3. 701 of the errors go away if we set declaration-casts: false. These are places where, once we have strong mode in the VM, we will be using as instead of assignments.

4. double x = 0.0.clamp(0.0, 0.0); doesn't work because double returns num.

5. Stream.firstWhere returns a Future<dynamic> instead of a Future<T>.

6. There's probably others. With 1668 errors to go through, I could only sample the problems, and 1, 2, and 3 above are very common indeed so they swamp out the others.

[matanlurey]

We (AngularDart) have been talking internally about altering the default set of checks for "stricter" strong-mode with @leafpetersen and @jmesserly - maybe we can collaborate on this instead of having separate requests to the strong-mode team.

[bwilkerson]

There is also issue #30397. It seems to me that the issue tracker is a good place to collaborate.

[srawlins]

We often have variables of type dynamic, which we then pass to methods that expect particular types. I think we want this to do a type check at runtime then cast automatically. The point of dynamic is that it opts you out of static typing.

With the release of null safety, I think you have exactly what you want, and implicit-casts: false (and the upcoming strict-casts: true) are unnecessary for Flutter:

• no implicit casts from a non-dynamic type to any other type are allowed.
• the only checks that implicit-casts: false add are during an implicit cast from dynamic.

I can close if this analysis mode is not interesting for the Flutter team.

[srawlins]

implicit-casts has landed. I recommend reviewing that flag and opening a new issue if it proves unhelpful in any way.