unnecessary_cast false positive when cast affects inferred type

[jamesderlin]

Describe the issue

x as T seems to trigger an unnecessary_cast lint if the static type of x is statically known to be a subtype of T. Although such a cast superficially seems pointless, there can be cases where the cast makes a difference. For example, consider:

var record = (1, 2, 3);

The inferred type of record is (int, int, int). If I want one of the members to be a num, I could explicitly specify the Record type:

(int, num, int) record = (1, 2, 3);

but that becomes awkward, tedious, and potentially error-prone if the Record has many members. I alternatively could do:

num x = 2;
var record = (1, x, 3);

which I suppose is okay, but it's more verbose and requires naming more things. I'd rather do:

var record = (1, 2 as num, 3);

The last version, however, triggers the unnecessary_cast lint (using DartPad based on Flutter 3.10.0-4.0.pre Dart SDK 3.0.0-431.0.dev).

To Reproduce

See above.

Expected behavior

Ideally the unnecessary_cast lint would trigger if and only if removing the cast has no effect.

I admit that this sort of situation probably isn't likely to occur much in practice.

[srawlins]

If I want one of the members to be a num,

Can you give an example of when you want this? There are many examples of unnecessary_cast false positives (#44411, #48984, #49269, #49550), but I'm looking for some code that becomes an error without that as num.

[eernstg]

class A<X extends A<X>> {}
class B extends A<B> {}
class C extends B {}

void f<X extends A<X>>(X x) {}

void main() {
  f(B()); // OK.
  f(C()); // Couldn't infer type parameter `X`.
  f(C() as B); // OK, but 'unnecessary_cast'.
}

[jamesderlin]

Oops, I didn't see those other issues since I was searching in the wrong repository.

Can you give an example of when you want this?

A trivial example:

void main() {
  var record = (1, 2 as num);
  record = (1, 3.1415);
}

It doesn't seem too far-fetched that such a re-assignment might come about in something like:

final recordStore = <(int, num)>[];

bool somePredicate(int x) => false;

void main() {
  var record = (0, 0 as num); // Default value.
  for (var r in recordStore) {
    if (somePredicate(r.$1)) {
      record = r;
      break;
    }
  }

  // Do something with the found `record`.
}