Extension type expression evaluation error when from closure parameter

[FMorschel]

Repro below (I could not find a way to remove more code than this).

import 'dart:typed_data';

void main() {
  var b = B(0);
  var list = [b];
  b.value2;
  list.forEach((b) {
    print(b.value2);
  });
}

extension type A._(Uint8List i) {
  A(int i) : this._(Uint8List.fromList([i]));

  int get value => i.elementAt(0);
}

extension type B._(A i) implements A {
  B(int i) : this._(A(i));

  bool get value2 => i.value == 0;
}

Steps:

1. In the above code, place a breakpoint on line 6 (b.value2) and line 8 (inside the closure)
2. Debug and add to watch b.value2
3. On the first break, you'll see true as the result (as expected).
4. On the second break, this is the output for expression evaluation:

Unhandled exception:
NoSuchMethodError: Class 'int' has no instance method 'elementAt'.
Receiver: 274334218320
Tried calling: elementAt(0)
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:38:5)
#1      A.value (package:bug/bug.dart:15:22)
#2      B.value2 (package:bug/bug.dart:21:24)
#3      Eval ()
#4      main.<anonymous closure> (package:bug/bug.dart:8:13)
#5      List.forEach (dart:core-patch/growable_array.dart:417:8)
#6      main (package:bug/bug.dart:7:8)
#7      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:297:19)
#8      _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)

---

CC @DanTup

[dart-github-bot]

Summary: The issue is that extension type expression evaluation fails when the extension type is used as a parameter in a closure. The evaluation throws a NoSuchMethodError when trying to access a method of the extension type within the closure.

[FMorschel]

Also, if you place a breakpoint on line 21 (bool get value2 => i.value == 0) and evaluate i.value you get that it is not defined. I think this is related to the problem.

evaluateInFrame: (113) Expression compilation error
org-dartlang-debug:synthetic_debug_expression:1:1: Error: The getter 'i' isn't defined for the class '_Uint8List'.
 - '_Uint8List' is from 'dart:typed_data'.
Try correcting the name to the name of an existing getter, or defining a getter or field named 'i'.
i.value
^

---

If you add to A the implementation:

extension type A._(Uint8List i) implements Uint8List {

When you evaluate on elementAt(0) (no i) at that line you get no errors. Maybe something like this is not being handled by the VM?

[mraleph]

cc @bkonyi

[FMorschel]

One more thing that I'm not sure it is related but it seems that this has lots of related problems:

Change B implementation for the following and add a breakpoint on the call for foo.

extension type B._(A i) implements A {
  factory B(int i) {
    foo();
    return B._(A(i));
  }

  static void foo() {}

  bool get value2 => i.value == 0;
}

When you evaluate foo you get:

evaluateInFrame: (113) Expression compilation error
org-dartlang-debug:synthetic_debug_expression:1:1: Error: Method not found: 'foo'.
foo()
^^^

If foo was a top level function with the same implementation you'd get null as a response.

[FMorschel]

One last thing (I think 🙃) I noticed this when trying to hover a getter I had defined (like value/value2 above). I'm not sure if fixing the evaluation will fix the hover as well (it shows the same as when not debugging - the base identification), but just to be explicit that is not working.

[bkonyi]

Thanks for the detailed report @FMorschel! Having a reproduction case is extremely helpful!

FYI @johnniwinther @jensjoha, I think this might be a CFE issue?

[bwilkerson]

Given that it happens when attempting to display a hover, I'm fairly sure it's an analyzer issue (either only or also).