Lint: Avoid using function types as a runtime `Type`

[pq]

From @matanlurey on November 13, 2018 1:29

I don't know how this lint request will interact with the generalized typedef language proposals

In Dart 2.x, typedef relates directly to its structural type, and is not a "type" itself. Which means:

typedef A = void Function();
typedef B = void Function();
void main() {
  var set = Set();
  set.add(A);
  set.add(B);
  print(set.length); // 1
}

Due to the severe restrictions of using mirrors in any real code, treating a typedef definition as a runtime Type is not useful in anyway, and can be very confusing/error prone to users (see above). There is some resistance to amending the style guide (dart-lang/site-www#1221), which I think is on point - so instead I opened this request.

With this lint, you'd see:

typedef A = void Function();
typedef B = void Function();
void main() {
  var set = Set();
  // LINT:
  set.add(A);
  //            ^
  // LINT: AVOID using a typedef or function type as a runtime type literal.
  set.add(B);
  //            ^
  // LINT: AVOID using a typedef or function type as a runtime type literal.
  print(set.length); // 1
}

I don't see any real downsides to this lint.

/cc @natebosch @munificent @leafpetersen @srawlins

Copied from original issue: #35144

[pq]

From @stereotype441 on November 13, 2018 23:44

@pq should this bug be moved into the linter repo?

[eernstg]

Interesting! This seems to be related to another topic that we've discussed from time to time, namely branded types. Here's a proposal for some support in that direction, with some remarks near the end about why it is related to this issue: #57828.