Cannot debug values inside some if-statements: [sentinel kind: OptimizedOut] from evaluateInFrame()

[vital-edu]

When I try to evaluate values within if statements in the debug console the following error appears:

[Sentinel kind: OptimizedOut, valueAsString: <optimized out>] from evaluateInFrame()
#0      DartDebugAdapter.evaluateRequest (package:dds/src/dap/adapters/dart.dart:995:7)
<asynchronous suspension>
#1      BaseDebugAdapter.handle (package:dds/src/dap/base_debug_adapter.dart:143:7)
<asynchronous suspension>

Snippet that reproduces the issue (add the breakpoints in the line below the comments).

void main() {
  var data = {
    'number': 20,
    'canProcess': true,
  };
  final worker = Worker(data);
  print(worker.result);

  final result = worker.maybeProcess(data, (data) {
    if (data is Map) {
      // Breakpoint 2: without issue
      return data['number'] % 2 == 0;
    }
    return null;
  });

  print(result);

  print(worker.getResult());
  print(dataHasEventNumber(data));
}

class Worker {
  final dynamic _data;

  Worker(this._data);

  bool? get result {
    return maybeProcess(_data, (data) {
      if (data is Map) {
        // Breakpoint 1: with issue
        return data['number'] % 2 == 0;
      }
      return null;
    });
  }

  bool? getResult() {
    return maybeProcess(_data, (data) {
      if (data is Map) {
        // Breakpoint 3: with issue
        return data['number'] % 2 == 0;
      }
      return null;
    });
  }

  bool? maybeProcess<String>(dynamic data, bool? Function(dynamic) fun) {
    if (data is Map && data['canProcess'] == true) {
      return fun(data);
    }

    return null;
  }
}

bool? dataHasEventNumber(data) {
  if (data is Map) {
    // Breakpoint 4: without issue
    return data['number'] % 2 == 0;
  }
  return null;
}

Tested on flutter master and flutter stable channels, using VSCode (printing the value in the Debug Console) or Android Studio (evaluating expression).

Master channel: Dart SDK version: 3.1.0-118.0.dev (dev) (Tue May 16 18:58:22 2023 -0700) on "macos_x64"
Stable channel: Dart SDK version: 3.0.1 (stable) (Tue May 16 11:57:19 2023 +0000) on "macos_x64"

This bug does not exist on Dart SDK version: 2.19.2 (stable) (Tue Feb 7 18:37:17 2023 +0000) on "macos_x64"

[a-siva]

//cc @derekxu16

[DanTup]

I've had this report from a number of VS Code users too (there's an open issue at Dart-Code/Dart-Code#4555). I can reproduce it in some cases (using functions not if statements), though only in Flutter apps.

Below is a Flutter app repro. It has two debugger() calls in two functions that are the same. One is in main() and hovering over aaa works fine. The second is the same in the build method, but the evaluations fail with "".

import 'dart:developer';

import 'package:flutter/material.dart';

void main() {
  (int aaa) {
    debugger();
    print(aaa); // hover over aaa WORKS
  }(1);

  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    (int aaa) {
      debugger();
      print(aaa); // hover over aaa DOES NOT WORK
    }(1);
    return const MaterialApp();
  }
}

Working VM request/response:

==> {
    "id": "695",
    "jsonrpc": "2.0",
    "method": "evaluateInFrame",
    "params": {
        "disableBreakpoints": true,
        "expression": "aaa",
        "frameIndex": 0,
        "isolateId": "isolates/708117027186575"
    }
}

<== {
    "jsonrpc": "2.0",
    "result": {
        "type": "@Instance",
        "_vmType": "Smi",
        "class": {
           // snip
        },
        "identityHashCode": 0,
        "kind": "Int",
        "fixedId": true,
        "id": "objects/int-1",
        "valueAsString": "1"
    },
    "id": "695"
}

Not working VM request/response:

==> {
    "id": "699",
    "jsonrpc": "2.0",
    "method": "evaluateInFrame",
    "params": {
        "disableBreakpoints": true,
        "expression": "aaa",
        "frameIndex": 0,
        "isolateId": "isolates/708117027186575"
    }
}

<== {
    "jsonrpc": "2.0",
    "result": {
        "type": "Sentinel",
        "kind": "OptimizedOut",
        "valueAsString": "<optimized out>"
    },
    "id": "699"
}

[starball5]

Related to this and Dart-Code/Dart-Code#4555 on Stack Overflow:

• VSCode debug
• Flutter message while debugging after upgrading to 3.10

Updated answer posted here

[DanTup]

@bkonyi any thoughts on this? It was noted a few times in #52574 but I believe it's a different issue. It occurs at breakpoints and happens with both debug adapters. I included a simple repro above.

[bkonyi]

I'm not sure why one case works but the other doesn't. Maybe @alexmarkov might know why aaa is optimized out in one case and not the other?

[alexmarkov]

@bkonyi This happens because build method is an instance method, and main is static.
The closure in the build method doesn't use this, so it is not captured in the closure context. But debugger requires this in order to evaluate expression in the context of an instance method:

sdk/runtime/vm/debugger.cc

Lines 1139 to 1142 in 8dfd804

	if (receiver.ptr() == Object::optimized_out().ptr()) {
	// Cannot execute an instance method without a receiver.
	return Object::optimized_out().ptr();
	}

This limitation was added in 79bcad6. Maybe we can relax that condition in case evaluated expression doesn't use this. /cc @rmacnak-google

[bkonyi]

Ah, that makes sense. Based on the number of 👍 on the issue, we should try and relax that condition if we can. Can you take a look at this @rmacnak-google?

[markalroberts]

Hi - any update on this? I believe downgrading to 3.7.12 may fix this, but I now have a few dependencies that need SDK>=3.0.0 and so I will need to downgrade these dependencies too...