No static warning for not void user-defined []= operator

[sgrekhov]

According to the Dart specification (10.1.1 Operators)

It is a static warning if the return type of a user-declared operator ‘[]=’ is
explicitly declared and not void.

In fact, there is an error in analyzer and no issues in VM

class C {
  int operator []=(index, value) => 42;
  operator[](index) {}
}

main() {
  C c = new C();
  c[0] = new C();
}

The program above produces
error - The return type of the operator []= must be 'void'
in analyzer and no errors or warnings in VM

Dart VM version: 2.9.0-13.0.dev (dev) (Fri May 29 15:59:05 2020 +0200) on "windows_x64"

[devoncarew]

@sgrekhov - it sounds like the analyzer has correct behavior here, and we'll need to update the VM / CFE to have the same behavior?

[sgrekhov]

@devoncarew specifications states 'static warning' but analyzer produces 'compile error'. @eernstg please confirm if it is correct

[eernstg]

This discrepancy has been known for a long time, cf. #36127 and #36126. Created dart-lang/language#1012 to settle the issue in the way that is based on discussions is the language team over time.

[johnniwinther]

Fixed in https://dart-review.googlesource.com/c/sdk/+/161165