Inferenced return type of generator function become nullable, if it contains `return;`

[chiholai-sanasofthk]

Dart SDK version: 3.5.4

If a generator function contains return , inferenced return type becomes Iterable<T?>/Stream<T?>, while it should be Iterable<T>/Stream<T>.

example code:

void main() {
  //==================== sync* ===============================

  //✅ inferenced return type is Iterable<int>
  withOutReturnSync() sync* {
    yield 1;
  }

  // ❌ inferenced return type becomes `Iterable<int?>`, expect to be `Iterable<int>`
  withReturnSync() sync* {
    yield 1;
    return;
  }

  //✅ manually specifying the type works fine
  Iterable<int> withReturnSync2() sync* {
    yield 1;
    return;
  }

  //===================== async* =================================

  //✅ inferenced return type is Stream<int>
  withoutReturnASync() async* {
    yield 1;
  }

  // ❌ inferenced return type becomes `Stream<int?>`, expect to be `Stream<int>`
  withReturnASync() async* {
    yield 1;
    return;
  }

  //✅ manually specifying the type works fine
  Stream<int> withReturnASync2() async* {
    yield 1;
    return;
  }
}

[bwilkerson]

@eernstg What is the correct behavior in this case?

[eernstg]

Good catch, @chiholai-sanasofthk!

@bwilkerson, I think the feature specification needs to be adjusted.

A local function declaration is subject to return type inference in the same way as a function literal with the trivial context type _ (specified here).

Function literal return type inference is specified here.

It does have the following rule:

For each return; statement in the block, update T to be UP(Null, T).

However, this item is irrelevant for generator functions because it will never cause the returned value to be null, it is just used to detect that the job done by the generator is complete (so we won't add any further elements to the iterable or stream).

I created this spec change proposal: dart-lang/language#4210.