Introduce intersection types in strong-mode analyzer?

[ochafik]

This would allow precise analysis of the following (real-world) case:

if (entity is Observable && entity is Map) {
  takeMap(entity);
  //      ^
  // [WARNING] The argument type 'Observable' cannot be assigned to the parameter type 'Map'.
}

cc/ @leafpetersen

(of course, the workaround here is to pass entity as dynamic, but it just feels wrong)

[bwilkerson]

Or reverse the order of the test:

if (entity is Map && entity is Observable) {

[ochafik]

Sure, that as well (it does feel less wrong :-) ), although the general case would be:

if (entity is Observable && entity is Map) {
  takeMap(entity);
  takeObservable(entity);
}

[floitschG]

This is something we would like to improve.
Similarly annoying:

if (o is! List) return null;
o[x];  // Not inferred to be of type List.

or

if (o.x is List) {
  o.x[0];  // Not inferred to be a List.
}

[ochafik]

@floitschG +1, first example filed as #26195 for the record

[lrhn]

Closing as duplicate of #25565 (enhanced type promotion).