"Did this work before? N/A" Apologies, I missed out filling that in. See my final bullet point: Fairly certain this is a recent regression.

Definitely scope chain stuff (a comment I made in the issue.js source), because if I add this inside the scoping function:

function bar() {
    return foo.apply(this, arguments);
}

...and then use `bar` instead of `foo`, the problem goes away. Attached.

Deleted: issue2.js 1.4 KB

issue2.js 1.4 KB View Download

It looks like V8 issue. I attached regression test for V8.
Yang, could you take a look?

Deleted: regress-621361.js 1.1 KB

regress-621361.js 1.1 KB View Download

Reduced repro:

var Debug = debug.Debug;

function listener(event, execState, eventData, data) {
  if (event != Debug.DebugEvent.Break) return;
  try {
    print(execState.frame().evaluate("x").value());
    print(execState.frame().sourceLineText());
    execState.prepareStep(Debug.StepAction.StepIn);
  } catch (e) {
    print(e, e.stack);
  }
}

Debug.setListener(listener);

(function() {
  debugger;
  var x = l => l;
})();

The bug is in the scope iterator. The break location in the assignment is at the start of the arrow function

var x = l => l;
        ^___ break

We parse and find nested scopes based on break location, and wrongly assume that we are inside the arrow function.

I have a fix coming up.

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/071b655fa90a3db1f41717f28cb50fe9f2c6db2c

commit 071b655fa90a3db1f41717f28cb50fe9f2c6db2c
Author: yangguo <yangguo@chromium.org>
Date: Wed Jul 27 09:01:03 2016

[debugger] Scope iterator should not visit inner function literals.

R=marja@chromium.org
BUG= chromium:621361 

Review-Url: https://codereview.chromium.org/2185913003
Cr-Commit-Position: refs/heads/master@{#38087}

[modify] https://crrev.com/071b655fa90a3db1f41717f28cb50fe9f2c6db2c/src/debug/debug-scopes.cc
[add] https://crrev.com/071b655fa90a3db1f41717f28cb50fe9f2c6db2c/test/mjsunit/regress/regress-crbug-621361.js

The fix is fairly simple. I'd like to merge it.

M52 is already in Stable for Desktop and bar is VERY high. We're going to have M52 refresh Stable release next week and we can take this change in ONLY if it is critical, baked/verified in Canary and safe to merge. Please confirm this. Thank you. 
(Seems like this issue is specific to Linux and already exists on M51, so I'm leaning towards NOT to take it for M52 unless it is absolutely critical).

This is not limited to Linux. The bug probably existed a lot longer, and only occurs with DevTools open. So I guess we can also wait, and only merge to M53.

Please remove "Merge-Request-52" label if  M52 merge is not needed per comment #11. Thank you.

[Automated comment] Request affecting a post-stable build (M52), manual review required.

Your change meets the bar and is auto-approved for M53 (branch: 2785)

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/bd1d4bb4d3e850a63e731de3a984802fa3dd1a6a

commit bd1d4bb4d3e850a63e731de3a984802fa3dd1a6a
Author: Yang Guo <yangguo@chromium.org>
Date: Thu Jul 28 11:03:16 2016

Merged: [debugger] Scope iterator should not visit inner function literals.

Revision: 071b655fa90a3db1f41717f28cb50fe9f2c6db2c

BUG= chromium:621361 
LOG=N
NOTRY=true
NOPRESUBMIT=true
NOTREECHECKS=true
TBR=hablich@chromium.org

Review URL: https://codereview.chromium.org/2187273002 .

Cr-Commit-Position: refs/branch-heads/5.3@{#29}
Cr-Branched-From: 820a23aade5e74a92d794e05a0c2b3597f0da4b5-refs/heads/5.3.332@{#2}
Cr-Branched-From: 37538cb2c1b4d75c41af386cb4fedbe5566f5608-refs/heads/master@{#37308}

[modify] https://crrev.com/bd1d4bb4d3e850a63e731de3a984802fa3dd1a6a/src/debug/debug-scopes.cc
[add] https://crrev.com/bd1d4bb4d3e850a63e731de3a984802fa3dd1a6a/test/mjsunit/regress/regress-crbug-621361.js