CFE missing compile time error: const keys/switch cases should not implement ==

[sigmundch]

The spec doesn't allow const values that implement == to be used as keys in const maps, or as cases on switch statements.

The CFE is currently not producing a compile-time error for it.

Here is an example:

class A {
  final x;
  const A(this.x);
  operator==(y) => x < y.x;
}

const one = const A(1);
const two = const A(2);

const map = const {one: 1, two: 2};

main() {
  print(map[one]);
  print(map[two]);
  print(one == two);
}

Then:

> dart a.dart
'/a.dart': error: line 11 pos 20: key value must not implement operator ==
const map = const {one: 1, two: 2};
                   ^

> dart --preview-dart-2 a.dart
2
null
true

I wanted to file a bug for this issue because it accidentally hides a problem in dart2js, that Type cannot be used in these const contexts (see #17207), the sooner we handle this, the lower the chance we'll make a breaking change later on.

[lexaknyazev]

While it's clear why an arbitrary class implementing operator == cannot be used as a compilation-time constant key, could TypeImpl be a special case? It uses a string _typeName which seems to be unique for each class (even when actual class names collide).

[dgrove]

With -dev.59, the Dart Analyzer produces these errors:

Analyzing /usr/local/google/home/dgrove/tmp/dart2stable/32557.dart...
error • The constant map entry key expression type 'A' can't override the == operator at tmp/dart2stable/32557.dart:10:20 • const_map_key_expression_type_implements_equals
error • The constant map entry key expression type 'A' can't override the == operator at tmp/dart2stable/32557.dart:10:28 • const_map_key_expression_type_implements_equals
2 errors found.