heisenbug

[kzc]

• Rollup Version: rollup v2.50.0
• Operating System (or Browser): n/a
• Node Version (if applicable): v16.1.0

Repro

2.50.0 REPL

$ cat heisenbug.js 
function heisenbug() {
    var a;
    function f(b) {
        if (a) console.log(a === b.c ? "PASS" : "FAIL");
        a = b.c;
    }
    function g() {}
    function h() {
        f({
            c: g
        });
    }
    h();
    h();
}
heisenbug();
heisenbug();

// if following line is commented out then rollup generates an empty chunk
//console.log(123);

Expected Behavior

$ cat heisenbug.js | node
PASS
PASS

Actual Behavior

$ cat heisenbug.js | rollup

- → stdout...

(!) Generated an empty chunk

This example worked in rollup 2.48.0.

[lukastaegert]

Thanks a lot for spotting this one! This was the result of the new lazy object deoptimization, which did not request new tree-shaking passes when an object was deoptimized. Fixed in #4111.

[kzc]

Glad you found the problem. This bug took quite a while to isolate.

[lukastaegert]

And I am glad you did. These kind of tests turn out to be useful time and again in the future when the logic is expanded or fundamentally changed.

[kzc]

Most of my recent bug reports were adapted from the uglify-js test suite, by the way. This one was particularly strange though. I initially thought it was a strict/non-strict mode thing.

All the failing non-strict mode tests were ignored as Rollup assumes it is always dealing with ES module inputs. I don't think that's of concern to most users.