Iterable<A>.toNonEmptyListOrNull() is broken for nullable A

[mjadczak]

The current implementation of this function is as follows:

public fun <A> Iterable<A>.toNonEmptyListOrNull(): NonEmptyList<A>? =
  firstOrNull()?.let { NonEmptyList(it, drop(1)) }

but when A is a nullable type, and the first element of the list is null, the function incorrectly returns null!

It's also probably not best practice to iterate the iterable twice, just in case it cannot be iterated multiple times, or it is costly to do so.

Here is a proposed replacement, which is admittedly less pretty, but doesn't have the same issue and only iterates the iterable once:

public fun <A> Iterable<A>.toNonEmptyListOrNull(): NonEmptyList<A>? {
    val iter = iterator()
    if (!iter.hasNext()) return null
    return NonEmptyList(iter.next(), Iterable { iter }.toList())
}

[nomisRev]

Hey @mjadczak,

Thank you for this bug report! 🙌 Ouch, nullability problems 🙈 So easy to sneak through when not using Option in generic context.

If you're interested in contributing this could be a great contribution!

[mjadczak]

@nomisRev submitted as #3127 - my first PR so please let me know if there's some extra bits I need to run (not sure if the various format and ABI checking runs as part of the normal CI)