dart2js issue with negating a value via an index from a list

[sethladd]

https://dartpad.dartlang.org/8a387639cb5fdb219ffe

main() {
  var a = [0.0];
  a[0] = -0.0;
  print("a[0] is ${a[0]} , is negative? ${a[0].isNegative}"); // "a[0]=-0.0 true"

  var b = [0.0];
  print(b); // [0.0]
  b[0] = -b[0];
  print(b); // [-0.0]
  print("b[0] is ${b[0]} , is negative? ${b[0].isNegative}"); // "b[0]=0 true" (?)
}

Output from Dart Pad (dart2js):

a[0] is -0.0 , is negative? true
[0]
[-0.0]
b[0] is 0 , is negative? true

Expected: b[0] is -0.0 , is negative? true

Actual: b[0] is 0 , is negative? true

---

Output from the VM:

a[0] is -0.0 , is negative? true
[0.0]
[-0.0]
b[0] is -0.0 , is negative? true

[rakudrama]

Consider

var x1 = 0;
var x2 = 0.0;

Under dart2js, 0 and 0.0 are represented by the same bits.
That means it must always be true that (x1 is int) == (x2 is int).
Since the run time test x2 is int == true, we analyze 0.0 as if it is an int.
Somewhere we are missing that in this world int is a subtype of double (because every JavaScript number is a double, but some represent integer values).
We also have a type inference rule that x is int ==> (-x) is int.
This leads us to an optimization that works for int values but not for doubles.
We can stringify an int value x via the idiomatic JavaScript expression ""+x.
This cannot be used for doubles since it prints -0.0 as 0.
We must either (1) make sure that -0.0 is not an int, and change the inference rule to contradict int.unary- to say the result is num, or (2) consider -0.0 to be an int and give up on generating idiomatic JavaScript.

Currently the expression confuse(-0.0) is int returns true.
To change this would require a much slower 'is int' test.
ES6 has a Number.isInteger test that also considers -0.0 to be an integer (but excludes infinities).
So it seems that we must be much more careful about generating idiomatic JavaScript for interpolation and track whether a value can be -0.0 to avoid the inconsistency.

There is a related bug with constant-folding the string representation of 0.0.
Since 0 and 0.0 are the the identical representation, dart2js should only ever print one of 0 or 0.0.
The following program prints both:

import 'package:expect/expect.dart';

@NoInline()
@AssumeDynamic()
confuse(x) => x;

main() {
  var z = 0.0;
  print('z = ${z}');
  print('confuse(z) = ${confuse(z)}');
  z = -z;
  z = -z;
  print('z = ${z}');
  print('confuse(z) = ${confuse(z)}');
}

z = 0
confuse(z) = 0
z = 0.0
confuse(z) = 0

Notice how the string is completely constant-folded, but differently:

    main: function() {
      P.print("z = 0");
      P.print("confuse(z) = " + H.S(T.confuse(0)));
      P.print("z = 0.0");
      P.print("confuse(z) = " + H.S(T.confuse(0.0)));
    }

Even more curious, this happens only for 0.0, not, say, 1.0.

[lrhn]

Bug still here. DartPad now uses DDC, which doesn't have the issue.

[rakudrama]

See #49870 for a more up-to-date analysis.
I think the best way to fix this is with a couple of breaking changes.