[NNBD] Type promotion fails for variable uses in anonymous functions

[nex3]

The following function:

int foo({int? arg}) {
  arg = 0;
  return (() => arg)();
}

Produces this error:

  error • A value of type 'int?' can't be returned from the function 'foo' because it has a return type of 'int'. • test.dart:3:10 • return_of_invalid_type

Despite the fact that arg will clearly always be non-null when it's returned. There's not even a hypothetical risk that arg will be reassigned to null between the function being created and being invoked, since the non-nullable assignment appears before the function is created and no nullable assignments exist anywhere.

[stereotype441]

Yeah, this is a known limitation of type promotion. When it sees an anonymous function, it makes a pretty conservative assumption about what might happen to any variables that anonymous function reads: it reasons that if the variable is ever written to, then it's possible that the variable's promotion will be invalidated by a write sometime between the time it's created and the time it's called. So to be on the safe side, it invalidates the type promotion.

Obviously in this example, that's overly conservative, for three reasons: (a) because the anonymous function is called exactly once, immediately after it's created, (b) because the only assignment to arg happens before the anonymous function is created, and (c) because that assignment always assigns a non-null value.

I believe we have some other bugs in the language repo discussing similar issues. I'll try to dig them up and link them here, so that we remember to think about this case when we think about future improvements to type promotion.

[stereotype441]

FWIW, you can work around this limitation by creating a temporary variable that's never written to:

int foo({int? arg}) {
  arg = 0;
  var tmp = arg;
  return (() => tmp)();
}

[nex3]

This is a pretty rough thing to have to work around—Dart heavily encourages the use of anonymous functions for things like collection manipulation, so added friction passing values in produces a lot of pain.

Obviously in this example, that's overly conservative, for three reasons: (a) because the anonymous function is called exactly once, immediately after it's created, (b) because the only assignment to arg happens before the anonymous function is created, and (c) because that assignment always assigns a non-null value.

For what it's worth, (a) here is basically a coincidence—in the actual code in which I'm seeing this issue, there's no way for the analyzer to know that the callback is only called once. But (b) and (c) are both usually true for these situations.

[renatoathaydes]

FYI the concept used in Java and Kotlin of effectively final might be useful.

For example, in Kotlin, this works fine:

fun main() {
    println(foo(true)())
}
fun foo(b: Boolean): () -> Int {
    var n: Number
    n = if (b) 1 else 0.3
    if (n is Int) {
        return { n }
    }
    return { 0 }
}

The closure can only accept n because, even though it's not final, it never changes and that can be safely determined by the compiler.

If you re-assign n at any point in the function AFTER the closure capturing it is created, it won't compile anymore:

fun foo(b: Boolean): () -> Int {
    var n: Number
    n = if (b) 1 else 0.3
    if (n is Int) {
        return { n } // Smart cast to 'Int' is impossible, because 'n' is a local variable that is captured by a changing closure
    }
    n = 4
    return { 0 }
}

Dart should use the same behaviour IMHO.

[renatoathaydes]

BTW In Kotlin, unlike Java, the variable can even be re-assigned as long as it is never re-assigned AFTER the closure captures it (and I didn't use an argument as in the Dart example because in Kotlin, arguments are always final).

[gazialankus]

I think implementing this one must be the easiest among the three reasons:

(c) because that assignment always assigns a non-null value

It's easier to explain away the two other cases with technical limitations but this one is a bit surprising.

FWIW I recently came across this in this example: https://dartpad.dev/?id=fc440d205d8855f5ebbc919a12eec8ea