Type promotion issue with nullable functions

[nielsenko]

Here is a condensed example of the problem.

typedef Func = int Function(int);
Func weird(Func? functor) {
  functor ??= (_) => 42; // functor cannot be null hereafter
  return (int i) {
    return functor!(i); // yet, why is ! needed here?
  };
}

Even moving the promotion inside the returned function fails:

typedef Func = int Function(int);
Func weird(Func? functor) {
  return (int i) {
    functor ??= (_) => 42; // functor cannot be null hereafter
    return functor!(i); // yet, why is ! needed here?
  };
}

But this works:

Func weird(Func? functor) {
  functor ??= (_) => 42; // functor cannot be null hereafter
  return functor; // <-- this works;
  // return functor!; // <-- this correctly warns that ! will have no effect
}

and this as well:

Func weird(Func? functor) {
  final f = functor ?? (_) => 42; // f cannot be null
  return (int i) {
    return f(i); // Tada 🎉 
  };
}

dart info

#### General info

- Dart 3.7.2 (stable) (Tue Mar 11 04:27:50 2025 -0700) on "macos_x64"
- on macos / Version 15.4 (Build 24E248)
- locale is en-DK

#### Project info

- sdk constraint: '^3.7.2'
- dependencies: 
- dev_dependencies: lints, test

[alexmarkov]

This is a language / front-end question, not specific to the VM.

/cc @stereotype441

[eernstg]

Note that this is not tied to function types:

Function foo(int? x) {
  x ??= 1;
  return () => x.isEven; // Error: .. the receiver can be null.
}

The reason why x isn't known to be non-null is probably that the expression occurs in the body of a function literal (also known as a lambda or a closure), and the flow analysis basically has to assume that the function literal can be executed at any time (so we can't know whether or not x ??= 1 has already occurred).

@stereotype441 would know whether there's an opportunity to reach a stronger conclusion, based on the location where that function literal occurs (that is, we might be able to know that x ??= 1 has occurred when () => x.isEven is invoked).

[stereotype441]

This is a known issue that comes up fairly frequently. I would love to work on fixing it, but it's a fairly big task and it hasn't yet become a high enough priority to tackle. The canonical issue is dart-lang/language#1536.