Map value for uninitialized key is different in DDC output vs Dart2js output

[corwinsheahan-wf]

dart --version
Dart VM version: 2.2.0 (Tue Feb 26 15:04:32 2019 +0100) on "macos_x64"

Hi There!

We're seeing in issue where the default value for a map lookup with a key that has not been initialized behaves differently in DDC output vs dart2js output. Here's a test which will pass when compiled via dart2js, but will fail when when compiled via DDC:

import 'package:test/test.dart';

main() {
  test('default param', () {
    // passes in dart2js and ddc
    expect(someFunc(null), null);

    final emptyMap = {};
    // fails in ddc, passes in dart2js
    expect(someFunc(emptyMap['should_be_null']), null);
  });
}

dynamic someFunc([c1 = 'some_default_value']) {
  return c1;
}

If you print out the value of emptyMap['should_be_null'], it is null on both DDC compiled output and dart2js compiled output. However, the return value in DDC from someFunc is actually some_default_value, and not null. One workaround is to pass the expression emptyMap['should_be_null'] ?? null to someFunc.

Could it be that emptyMap['should_be_null'] is undefined in the DDC output, but is explicitly null in the dart2js output?

[jakemac53]

cc @jmesserly

[jmesserly]

This is a known issue. This should be addressed in DDC's map implementations, which may or may not have a measurable performance impact on access times (I suspect it will not make much difference).

[jmesserly]

https://dart-review.googlesource.com/c/sdk/+/94920

[rakudrama]

I believe the issue is not that map returns null or undefined, but how DDC and dart2js differ in the defaulting of optional arguments.
There are many ways of getting a JavaScript undefined value, consider new List(100000)[2].

[jmesserly]

Would you mind opening another issue about that, if you'd like to discuss further? I'd rather keep this issue focused on the report, and discuss bigger design questions about calling conventions elsewhere.

(In general, DDC tries to avoid common pitfalls related to undefined, by using null where it is practical to do so. There are indeed cases where it breaks down, though these seem rare in practice. I don't think changing the calling convention in DDC is practical, but if you'd like to discuss further, we can certainly revisit it.)

[rakudrama]

@jmesserly Done: #36116