bug using redirecting constructor in initializer of extension type

[curt-weber]

Extension types don't seem to support typed data, for instance Float64x2.
There is a restriction that typed_data isn't extended, but that applies to other types like int, double, which extension types can use.

import 'dart:math';
import 'dart:typed_data';

extension type const Point._(Float64x2 point) implements Float64x2 {
  Point(double x, double y) : point = Float64x2(x, y);
  double get distance => sqrt(point.x * point.x + point.y * point.y);
}

void main() => print(Point(0, 0)); // prints null

I was checking to see if extension types would be more performant in the case of Offset, but using List<double> is actually slower from my tests.

[lrhn]

That sounds like a bug. You should be able to subtype typed data with extension types.

[srawlins]

Are you running this with DDC? For my part, this code:

import 'dart:math';
import 'dart:typed_data';

extension type const Point._(Float64x2 point) implements Float64x2 {
  Point(double x, double y) : point = Float64x2(x, y);
  double get distance => sqrt(point.x * point.x + point.y * point.y);
}

void main() {
  print('Float64x2: ${Float64x2(0, 0)}');
  print(Point(0, 0));
  print('Point: ${Point(0, 0)}');
}

prints this with the Dart VM (3.3.0):

Float64x2: [0.000000, 0.000000]
[0.000000, 0.000000]
Point: [0.000000, 0.000000]

and prints this in DartPad (DDC):

Float64x2: [0, 0]
null
Script error.

with the following exception in the console:

Uncaught TypeError: Cannot read properties of undefined (reading 'Symbol(dartx.toString)')
    at Object.strSafe (operations.dart:1072:38)
    at Object.main$0 [as main] (<anonymous>:69:33)
    at Object.main$ [as main] (<anonymous>:47:10)
    at <anonymous>:106:26
    at Object.execCb (require.js:5:16727)
    at e.check (require.js:5:10499)
    at e.<anonymous> (require.js:5:12915)
    at require.js:5:1542
    at require.js:5:13376
    at each (require.js:5:1020)
    at e.emit (require.js:5:13344)
    at e.check (require.js:5:11058)
    at e.enable (require.js:5:13242)
    at e.init (require.js:5:9605)
    at a (require.js:5:8305)
    at Object.completeLoad (require.js:5:15962)
    at HTMLScriptElement.onScriptLoad (require.js:5:16882)

suggesting some weird things: Point(0, 0) acts like null, except it is a value that doesn't even have a toString() method on it?

[cmkweber]

Yes, I should have mentioned that was in DartPad. Initially was trying to add it to a List<Point> but that wasn’t working and not showing the exception in DartPad for some reason.

[curt-weber]

Should this get moved to the sdk repo as a bug?

[sigmundch]

It appears this is triggering issues on both web compilers, coincidentally. Likely these are different bugs.

Both trigger when the initializer of the extension type representation field uses a redirecting factory constructor (which Float64x2 happens to have). Here is an standalone repro of this:

class B implements A {
  final int x = 1;
}
abstract class A {
  int get x;
  factory A() = B;
}

extension type C._(A point) implements A {
  C() : point = A(); // << initializer is redirecting factory
}

void main() {
  print(A().x);
  print(C().x);
}

This program compiles in DDC, but like above gets null instead of an instance of B when calling C(). Changing the initializer to use B() instead of A() works as expected.

Dart2js surprisingly crashes! Here is the crash error for the same program:

Internal Error: The compiler crashed when compiling this element.
  factory A() = B;
          ^
The compiler is broken.

When compiling the above element, the compiler crashed. It is not
possible to tell if this is caused by a problem in your program or
not. Regardless, the compiler should not crash.

The Dart team would greatly appreciate if you would take a moment to
report this problem at http://dartbug.com/new.

Please include the following information:

* the name and version of your operating system,

* the Dart SDK build number (3.4.0-179.0.dev), and

* the entire message you see here (including the full stack trace
  below as well as the source location above).

The compiler crashed: Null check operator used on a null value
#0      ClosureDataBuilder.createClosureEntities.processModel (package:compiler/src/js_model/closure.dart:375:59)
#1      ClosureDataBuilder.createClosureEntities.<anonymous closure>.<anonymous closure> (package:compiler/src/js_model/closure.dart:436:9)
#2      DiagnosticReporter.withCurrentElement (package:compiler/src/diagnostics/diagnostic_listener.dart:159:15)
#3      ClosureDataBuilder.createClosureEntities.<anonymous closure> (package:compiler/src/js_model/closure.dart:435:17)
#4      _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:633:13)
#5      ClosureDataBuilder.createClosureEntities (package:compiler/src/js_model/closure.dart:434:19)
#6      JClosedWorldBuilder.convertClosedWorld (package:compiler/src/js_model/js_world_builder.dart:172:41)
#7      JsBackendStrategy.createJClosedWorld (package:compiler/src/js_model/js_strategy.dart:184:52)
#8      Compiler.closeResolution (package:compiler/src/compiler.dart:743:25)
#9      Compiler.computeClosedWorld (package:compiler/src/compiler.dart:385:9)
#10     Compiler.produceClosedWorld (package:compiler/src/compiler.dart:534:21)
#11     Compiler.runSequentialPhases (package:compiler/src/compiler.dart:643:39)
<asynchronous suspension>
#12     Compiler.runInternal.<anonymous closure> (package:compiler/src/compiler.dart:317:7)
<asynchronous suspension>
#13     Compiler.runInternal (package:compiler/src/compiler.dart:316:5)
<asynchronous suspension>
#14     Compiler.run.<anonymous closure> (package:compiler/src/compiler.dart:237:11)
<asynchronous suspension>
#15     compile.<anonymous closure> (package:compiler/compiler_api.dart:255:30)
<asynchronous suspension>
#16     compile.compilationDone (package:compiler/src/dart2js.dart:730:3)
<asynchronous suspension>
#17     main (package:compiler/src/dart2js.dart:1240:3)
<asynchronous suspension>

[nshahan]

cc @johnniwinther

This looks like an issue with the lowering of redirecting factory constructors for both js compilers. @sigmundch example above looks like a good reproduction of the problem.

[fishythefish]

#55079 is likely the same bug in dart2js.

[sigmundch]

Moving temporarily to area-frontend, we can move it back if in fact it's an issue on the web compilers.

[chloestefantsova]

I might have a solution. I'm working on it in https://dart-review.googlesource.com/c/sdk/+/357161.

[sigmundch]

Thanks so much for the fix @chloestefantsova!

@chloestefantsova @johnniwinther - since the fix is fairly contained, it seems it may be worth considering filing a cherry-pick request for it. What do you think?

[chloestefantsova]

@chloestefantsova @johnniwinther - since the fix is fairly contained, it seems it may be worth considering filing a cherry-pick request for it. What do you think?

That's a good idea. I've just filed #55194.