allowInterop is no longer idempotent in DDC in dev build

[greglittlefield-wf]

Calling allowInterop on the return value of allowInterop used to return the same function but it doesn't in DDC in the dev build of the SDK.

Reproduced with

• Dart SDK 2.6.0-dev.0.0
• build_web_compilers 2.3.0

Reduced test case:

function() {}
final once = allowInterop(function);
final twice = allowInterop(once);
print(identical(once, twice)); // prints false

This issue does not reproduce in dart2js, or in 2.5.0 DDC/dart2js.

allowInterop implementation and compiled result, for reference

DDC source code for allowInterop:

sdk/sdk/lib/_internal/js_dev_runtime/lib/js/dart2js/js_dart2js.dart

Lines 557 to 570 in cff9c58

	F allowInterop<F extends Function>(F f) {
	var ret = _interopExpando[f];
	if (ret == null) {
	ret = JS(
	'',
	'function (...args) {'
	' return #(#, args);'
	'}',
	dart.dcall,
	f);
	_interopExpando[f] = ret;
	}
	return ret;
	}

DDC-compiled code for allowInterop:

  js.allowInterop = function allowInterop(F, f) {
    let ret = js._interopExpando._get(f);
    if (ret == null) {
      ret = function(...args) {
        return dart.dcall(f, args);
      };
      js._interopExpando._set(f, ret);
    }
    return F._check(ret);
  };

Could the issue be that the F._check check on the return value is creating a new instance that the expando doesn't know about? Edit: never mind, I was thinking about the Expando wrong. It stores the converted function on f, not ret.

[mraleph]

/cc @vsmenon

[vsmenon]

@natebosch - I don't think we handle this case.

[greglittlefield-wf]

Any idea if this behavior will be restored before release? I'm just asking so I know whether we should plan on updating code to not rely on the old behavior.

[natebosch]

I plan on either fixing this or rolling back the change that caused it before we go to stable.

[greglittlefield-wf]

👍 Awesome, thank you!