[cfe] Extension on type `void` can be called

[sgrekhov]

According to the extension methods specification

It is a compile-time error if the static type of the argument expression (expr) of an explicit extension invocation is void. (Expressions of type void are only allowed in a few specific syntactic positions, and the new explicit extension invocation object position is not included in those.)

But the following code produces an error in the analyzer but no any issues in CFE

extension ExtVoid on void {
  void checkme() {
    print("I'm extension");
  }
}

main() {
  ExtVoid(print("print() returns void")).checkme(); // Analyzer error. use_of_void_result
}

When run this program prints I'm extension. But there should be a compile-time error.

Dart SDK version: 3.7.0-123.0.dev (dev) (Fri Nov 8 08:06:44 2024 -0800) on "windows_x64"

[eernstg]

This is a tricky one! First, the current rules about extension declarations are in the language specification, so we'd need to find the relevant rules there.

The language specification does mention in commentary that it is a compile-time error if e has type void in an explicit invocation like ExtVoid(e).checkme(), and it does not directly mention the case where the on-type of the extension is void.

Moreover, it is specified explicitly that e in an expression like ExtVoid(e).checkme() must be assignable to the instantiated on-type (which is void in this case). But this situation is not specified to be yet another situation where an actual argument is passed to a formal parameter (that is, ExtVoid(e) is not a function invocation).

Based on the fact that this construct is a special case, not just another function invocation or any other pre-existing thing, I agree that there is no special exception here. So we should indeed have that error message, even when the extension is on void.

[lrhn]

That makes sense as specified.
We could choose to change it, so you can do an extension on void and instantiate it with a void-typed expression, and then the methods can't use this for anything since it has type void, so everything should be fine.

I just don't see any need for that. Make it an extensin on void and pass in null instead of an expression with type void, and everything should be the same.