LinearSeq#head and LinearSeq#tail aren't abstract

[argv-minus-one]

The LinearSeq#isEmpty, LinearSeq#head, and LinearSeq#tail methods should be abstract, so that it is obvious (and verified at compile time) that these methods must be implemented in order to have a working LinearSeq. Currently, they are not, and new LinearSeq[SomeType] {} is permitted at compile time but causes a stack overflow at run time.

Also, the LinearSeq documentation no longer says that these methods must be implemented. In 2.12 and earlier, this was in the documentation for LinearSeqOptimized, but it doesn't seem to have been brought over.

[dwijnand]

Also, the LinearSeq documentation no longer says that these methods must be implemented. In 2.12 and earlier, this was in the documentation for LinearSeqOptimized, but it doesn't seem to have been brought over.

This part can be fast-tracked, without issues of binary compatibility.

[SethTisue]

volunteer to tackle this for 2.13.2?

[guizmaii]

Is this related to the fact that the optimizer can't inline such code:

implicit final class RichList[A](private val list: List[A]) extends AnyVal {
  def sumBy[B: Numeric](f: A => B): B = list.foldLeft(Numeric[B].zero) { case (acc, a) => Numeric[B].plus(acc, f(a)) }
}

If I compile with the optimizer enabled and the -opt-warnings:any-inline-failed flag, I have the following error:

[error] Failed to check if scala/collection/LinearSeqOps::foldLeft(Ljava/lang/Object;Lscala/Function2;)Ljava/lang/Object; can be safely inlined to LanguageExtensions$RichList$ without causing an IllegalAccessError. Checking instruction INVOKEINTERFACE scala/collection/LinearSeq.isEmpty ()Z (itf) failed:
[error] The method isEmpty()Z could not be found in the class scala/collection/LinearSeq or any of its parents.
[error]     def sumBy[B: Numeric](f: A => B): B = list.foldLeft(Numeric[B].zero) { case (acc, a) => Numeric[B].plus(acc, f(a)) }
[error]                                                                          ^
[error] LanguageExtensions.scala:114:93: scala/collection/LinearSeqOps::foldLeft(Ljava/lang/Object;Lscala/Function2;)Ljava/lang/Object; could not be inlined:

??? 🤔

[som-snytt]

@guizmaii it won't inline methods that can be overridden. Here, I think it's looking for candidates to inline and notices that there is an abstract isEmpty in a subinterface (LinearSeqOps <: IterableOnceOps or whatever).

It's just looking at bytecode, so it doesn't have enough information about foldLeft to know that isEmpty is called on List there.

[SethTisue]

Not sure whether to leave this open — the linked PR initially aimed to change the code, but ended up changing the documentation only.

[som-snytt]

I created an issue on library-next for later revisitation.