Key in a constant map can't override the `==` operator, or `hashCode`DRT-W1273

The analyzer produces this diagnostic when the class of object used as a key in a constant map literal implements either the == operator, the getter hashCode, or both. The implementation of constant maps uses both the == operator and the hashCode getter, so any implementation other than the ones inherited from Object requires executing arbitrary code at compile time, which isn't supported.

Examples

The following code produces this diagnostic because the constant map contains a key whose type is C, and the class C overrides the implementation of ==:

class C {
  const C();

  bool operator ==(Object other) => true;
}

const map = {C() : 0};

The following code produces this diagnostic because the constant map contains a key whose type is C, and the class C overrides the implementation of hashCode:

class C {
  const C();

  int get hashCode => 3;
}

const map = {C() : 0};

Common fixes

If you can remove the implementation of == and hashCode from the class, then do so:

class C {
  const C();
}

const map = {C() : 0};

If you can't remove the implementation of == and hashCode from the class, then make the map non-constant:

class C {
  const C();

  bool operator ==(Object other) => true;
}

final map = {C() : 0};