document that `no_duplicate_case_values` is a no-op in Dart 3.0+

[pq]

https://github.com/dart-lang/language/blob/master/working/0546-patterns/patterns-feature-specification.md#switch-expression

[pq]

If I understand the exhaustiveness proposal right, unreachable cases will be treated as errors so this lint should not fire and there should be nothing to do here.

[pq]

@munificent: is this right?

[srawlins]

The no_duplicate_case_values example text has this test case:

  const String A = 'a';
  String v = 'aa';

  switch (v) {
    case 'aa':  // OK
    case 'bb':  // OK
    case A + A: // LINT
    case 'bb':  // LINT
    case A + 'b': // OK
    default:
  }

The exhaustiveness spec talks about literal or constant matcher:

We lift other constants to an extract pattern whose type is a synthesized subtype of the constant's type based on the constant's identity.

This means you don't get exhaustiveness checking for constants, but do get reachability checking:

String s = ...
switch (s) {
  case 'a string': ...
  case 'a string': ...
}

Here, there is an unreachable error on the second case since it matches the same string constant. Also the switch has a non-exhaustive error since it doesn't match the entire String type.

no_duplicate_case_values works by evaluating case expressions as constants, and then putting them in a Map and checking whether a case value is already in the Map. This may not be the same as identity... I'm not sure...

Anyhow, even if there is a difference, maybe we can claim its not interesting.

[lrhn]

Another thing to worry about with generalized pattern matching: Cases also get when clauses which can change the behavior of the case.
Say:

switch (e) {
  case 1 when random.nextBool():  print("1/1");
  case 1: print("1/2");
}

which is a kind-of silly way to randomly select between two cases, but it's allowed.

Safest aproach is probably to not apply this lint to any case with a when clause. (Such a case would probably also be ignored for exhaustiveness checking.)

[pq]

As implemented, this lint will not fire on SwitchPatternCases so it won't report anything on 3.0 ASTs. (It will continue to work for libraries that are opted back though.) We should update the docs to make this clear.