Expression compilation inside extension sometimes has #this not as the first parameter (resulting in bad stuff)

[jensjoha]

Say I have this code:

import 'dart:developer';

extension on String {
  List<String> get foo {
    List<String> result = [];
    int offset = indexOf(':') + 1;
    void bar() {
      debugger();
      while (offset < length) {
        if (codeUnitAt(offset) != 32) {
          return;
        }
        offset++;
      }
    }

    bar();
    result.add("$offset");
    offset++;
    return result;
  }
}

void main(List<String> args) {
  print(args[0].foo);
}

and say I run it like

dart --enable-vm-service t.dart hello

If I open up observatory (ehh, not built in any more but still possible) and go to the debugger (that is now paused) it says

<library>	t.dart
offset	0
#this	'hello'

Notice how #this is below offset.

Trying to evaluate offset it will say hello:

$ p offset
= 'hello'

which clearly isn't right.

Trying to run as

dart -DDFE_VERBOSE=true --enable-vm-service t.dart hello

confirms the wrong order too:

[offset, #this]

I've tried with current HEAD, Dart 3.8.0, Dart 3.7.2, Dart 3.6.2, Dart 3.5.4, Dart 3.3 and Dart 2.7.2 and they all behave the same.

[biggs0125]

@jensjoha @alexmarkov @johnniwinther The VM doesn't promise to always pass the receiver as the first parameter. Can we use the name to identify which parameter is the receiver here rather than the position?

We can look into making this consistent on the VM but if there's a simpler solution here that would be preferred.