Confusing warning when improperly typed closure passed to map

[stereotype441]

The following ill-typed code:

    void f(List<String> x, List<String> y) {
      x.addAll(y.map((String z) => 1.0));
    }

produces the strong mode warning Unsound implicit cast from Iterable<dynamic> to Iterable<String>.

This is confusing because there's nothing dynamic in the above code. I would expect one of the following behaviors:

• (String z) => 1.0 is inferred to have type (String) -> double, therefore y.map((String z) => 1.0) is inferred to have type Iterable<double>, but x.addAll requires Iterable<String>, therefore the message is Unsound implicit cast from Iterable<double> to Iterable<String>.
• Since x.addAll requires Iterable<String>, the type argument to y.map is inferred to be String, therefore y.map requires type (String) -> String. But the type of (String z) => 1.0 is (String) -> double, therefore the message is unsound implicit cast from (String) -> double to (String) -> String.

[jmesserly]

The failure mode of type inference with type parameters is to leave it with the implicit instantiation (the upper bound, usually dynamic but can be another type, e.g. num if Rectange<T extends num>).

Type of map is <S>(T -> S) -> Iterable<S>. Inference fails because double <: S and S <: String and no type can satisfy that. We know inference failed; if we wanted we could even list the constraints. But strong mode inference is not supposed to generate errors, in the current design. It only fixes them :)

We have discussed some kind of flag like "report inference failures" (#26781) that would change this behavior. But it's not clear if that's the right answer.

We also have no-implicit-casts flag now, which would make that cast failure into an error, and no-implicit-dynamic, which helps find implicit dynamic instantiations.

[jmesserly]

CC @leafpetersen ... I guess the TL;DR from my message is, we want to make this better, but current behavior is by design. It might make sense to merge this into #26781 .

(Another related bug is https://github.com/dart-lang/sdk/issues/25490 to finish implementing inference.)

[bwilkerson]

IMO, it would be much better from a user's perspective to pick one inference direction as the dominant direction and produce one of the two messages Paul was expecting. The current failure mode makes it hard for users to understand what the real problem is.

We have discussed some kind of flag like "report inference failures" (#26781) that would change this behavior. But it's not clear if that's the right answer.

I think it's the wrong answer. The tools should produce a clear and understandable error message without having to be run again with certain flags enabled.

We also have no-implicit-casts flag now, which would make that cast failure into an error, and no-implicit-dynamic, which helps find implicit dynamic instantiations.

My expectation is that those are either going to become the default behavior, or will be dropped, but in either case there should (eventually) only be one behavior.

[jmesserly]

IMO, it would be much better from a user's perspective to pick one inference direction as the dominant direction and produce one of the two messages Paul was expecting. The current failure mode makes it hard for users to understand what the real problem is.

absolutely. The thing to understand is right now we don't issue an error, so we're talking about introducing an error where one does not currently exist. Right now you're totally allowed to write:

Iterable<dynamic> iter = y.map<dynamic>((String z) => 1.0));
List<String> list;
list.addAll(iter); // we suspect a cast failure here, but aren't sure

Does that make sense? There's a warning because we suspect (but don't know for certain) a cast failure at runtime.

But we're all in agreement this is suboptimal. It's quite an interesting balancing act trying to get to a stronger type system from where we started in Dart 1 :)

I think it's the wrong answer. The tools should produce a clear and understandable error message without having to be run again with certain flags enabled.

My expectation is that those are either going to become the default behavior, or will be dropped, but in either case there should (eventually) only be one behavior.

Absolutely, 200% agree! The flags aren't a long term solution. We are trying to find our way to a "stronger mode" without breaking folks. It's not easy to figure out how to get there from where we're at. It may seem obvious what to do in this particular example, but tightening the system can also cause working code to be rejected. That's why it's a balancing act and some exploration is needed...

[bwilkerson]

To be clear, I wasn't suggesting that we not explore the possibilities, only that we not rely on a flag that might disappear as an answer to this problem. (Nor was I assuming that you were proposing doing so.)

Yes, it is hard to deal with backward compatibility. I find that sometimes it helps me do so if I know what the end goal is, hence my attempt to articulate the end goal. But I have no delusions that even knowing the end goal makes it easy to get there. :-)

[jmesserly]

Makes sense. The end goal (from chats with @leafpetersen) is to not have implicit casts. In which case having a context type (here, Iterable<String>) will require us to either be able to type the expression as Iterable<String> or issue an error that we cannot.

Once we're finishing inference (https://github.com/dart-lang/sdk/issues/25490) we'll be able to make the message more precise as well. Ideally downwards inference is driving the error message (it can narrow the error better), but that'd be tricky to express in the current (incomplete) implementation.

[leafpetersen]

The main issue here is about what to do when inference fails. We currently choose to simply fall back to dynamic. I think this is not ideal - this makes the results of inference extra confusing because it tends to just punt off the error to some other location. This is what happens here: we fall back to something that can't possibly work. Often the result will be runtime error - here you just get a confusing static warning.

I would prefer that we just issue an error in this case. This was the main thing that I wanted out of --no-implicit-dynamic. This one is extra bad, because it's an inference failure that results from being over-constrained rather than under-constrained. I think it should be the case that in all non-trivial (i.e. non-null cases), if inference is over-constrained you are guaranteed to get a runtime error. So we probably really should make at least that case always an error. The under-constrained case is less clear cut, since it's possible that dynamic will work.

Also, just to clarify - strong mode inference in general can introduce new static errors, but currently there are no errors that are specifically about inference (e.g. that inference has failed).

[jmesserly]

yeah, if we're willing to go there with strong mode/dart2, it's easy to do. We have all the info in hand at the point of inference:

sdk/pkg/analyzer/lib/src/generated/type_system.dart

Line 1340 in cdae2f2

// Inference failed. Bail.

just need to flow in an error reporter. The calling code will typically have one in hand.

NOTE: unfortunately we call this during resolution. This means it will hit the existing Analyzer bug, where errors from resolution are dropped on the floor if you call context.computeErrors and context.resolveCompilationUnit in the wrong order :( :( :(