Set<T> should allow looking up an element

[kasperl]

There's is no way to look up an element in a Set<T>. I want to be able to write something like this:

   class X implements Hashable {
     int x;
     X(this.x);
     operator ==(other) => (other is X) && (x == other.x);
     hashCode() => x;
   }

   final x0 = new X(42);
   final x1 = new X(42);
   assert(x0 == x1 && x0 !== x1);
   final set = new Set<X>();
   set.add(x0);
   assert(set.contains(x0) && set.contains(x1));
   assert(x0 === set.lookup(x0) && x0 === set.lookup(x1));

Not sure if lookup is the best method name, but getting access to the set element that tests equal to another given object is very useful.

[DartBot]

This comment was originally written by drfibonacci@google.com

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Added Triaged label.

[DartBot]

This comment was originally written by alexey.v.varla...@gmail.com

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Generally speaking, there is no point to look up an element in Set, plain check via contains() is idempotent. The use case you describe is rather filtering, i.e. you want to extract some element(s) from a Set, by characteristics which are orthogonal to equivalence semantics of a particular Set (e.g. to find an object by identity in a HashSet). There is a Collection.filter() feature already available to do this task:
assert(!set.filter(bool(el)=> el === x0;).isEmpty());

I would consider this issue as one more example of confusion caused by underspecified Set semantics (see issue #472).

[DartBot]

This comment was originally written by @seaneagan

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I think issue #3948 would be a nice solution to this.

[lrhn]

We are not planning to add methods that distinguishes otherwise equal objects, so looking up an object by an equal object should be equivalent to just returning the other object.
If you really care about which instance of equal objects you have in your set, either use a Map instead (with same object as both key and value), or use set.firstWhere((x) => x == other), which is going to be slower, but will get you the object.

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Added NotPlanned label.