Compiler - `[]=` Return-Type False-Positive Error

[RohitSaily]

dart --version: Dart SDK version: 3.10.0-247.0.dev (dev) (Thu Sep 25 13:09:08 2025 -0700) on "macos_x64"

Simple repro

final class AliasUser
{	Void operator []=(Object key, Object value){}
}
typedef Void=void;

---

The compiler emits the following error

main.dart:2:3: Error: The return type of the operator []= must be 'void'.
Try changing the return type to 'void'.
{       Void operator []=(Object key, Object value){}
        ^

I believe an alias of void should be acceptable as the return-type annotation for []=. Static analysis via VSCode does not report a warning. Only the compiler does.

[bwilkerson]

@lrhn @eernstg

[lrhn]

This is a bug in the compiler.

The requirement for setters and []= is that their return type must be void. You can omit a return type, but if you don't, it must be void.

This code writes a return type and the return type is void. It should not have any error.

It's not a syntactic requirement. There should be no place where a pure type alias for a non-generic type behaves differently from it's expansion as a type.