List.unmodifiable should not accept an iterable of any type

[navaronbracke]

Recently I bumped into a bug where some code was using a lot of List.unmodifiable(someIterable) to assign an unmodifiable list to a List<T>. However, since List.unmodifiable accepts an Iterable<dynamic> a type bug slipped through.

This bug would have been prevented if the signature of List.unmodifiable was List<T> List.unmodifiable(Iterable<T> input).

Is there a specific reason why this type argument is the way it is?

Dart SDK version: 3.0.5 (stable) (Mon Jun 12 18:31:49 2023 +0000) on "macos_x64"

[vsmenon]

I believe this is intended to allow you to tighten the type based on what you know about the elements. E.g.,:

  var list1 = json.decode('[1, 2, 3]');  // List<dynamic>
  var list2 = List<int>.unmodifiable(list1);  // Triggers runtime checks to tighten to List<int>

[lrhn]

That was the intuition. It was still a bad idea, born out of the unsafely type Dart 1, and that badness is the reason we introduced List.of as a better alternative to List.from.

We could either introduce an List.unmodifiableOf with better typing, risk a breaking change, or (my preference, even if it might have a much longer time frame) try to introduce unmodifiable list literals, say u#[v1, ... vs, v3].