Wrong order of evaluating map entries

[sgrekhov]

https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Control-flow-collections/Null-aware-elements/runtime_A02_t02.dart fails at run time on VM, dart2js and dart2wasm.

String _log = "";

int? f(int? v) {
  _log += "f($v);";
  return v;
}

main() {
  var map1 = {
    f(1): ?f(2),
    ?f(3): ?f(null),
    ?f(4): ?null
  };
  Expect.mapEquals({1: 2}, map1);
  Expect.equals("f(1);f(2);f(3);f(null);f(4);", _log); // Expect.equals(expected: <f(1);f(2);f(3);f(null);f(4);>, actual: <f(2);f(1);f(3);f(null);f(4);>) fails.
...
}

Why map value is evaluated before the key? According to the spec the key should be evaluated first.

• Else, if element is a nullAwareMapElement with entry k: v:

• Evaluate k to a value kv.

• If element has a null-aware key and kv is null, then stop. Else continue...

• Evaluate v to a value vv.

• If element has a null-aware value and vv is null, then stop. Else continue...

• Append an entry kv: vv to result.

[dart-github-bot]

Summary: The code in runtime_A02_t02.dart fails because the map values are evaluated before the keys, violating the specification which states that keys should be evaluated first. This discrepancy in evaluation order leads to incorrect map entries and a failed assertion.

[lrhn]

Nice optimization! Not sound, but nice thought. 😉

If ?f(2) would be null, the value of f(1) isn't needed, otherwise it's definitely needed, so we can just call it afterwards.
That only works if the functions have no side-effect. With side effects, and promotion from left to right, the key expression must be evaluated first, whether the value expression bails out or not.

Soundness example:

int? ifEven(int x) => x.isEven ? x : null; // x == null here!
void main() {
  int? x = null as dynamic;
  var map = {(x ??= 0) : ?ifEven(x)?.toRadixString(16)};
  print(map); // prints {} ?!?
}

which fails with

NoSuchMethodError: The getter 'isEven' was called on null.
Receiver: null
Tried calling: isEven
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:38:5)
#1      ifEven (file:///home/lrn/dart/sdk/dev/nae.dart:2:25)
...

which isn't sound.

Marking as front-end, since it affects multiple backends.

[eernstg]

Good catch!