Looks like 3.7.4-RC1 is DOA.

Minimalization of the issue, we'll release an RC2 next week with fix for this issue - we'll revert the problematic PR

package scala
package test
import scala.language.`2.13`
import language.experimental.captureChecking
trait IterableOnce[+A] { this: IterableOnce[A]^ =>
 def iterator: Iterator[A]^{this}
}
class SortedMap[K, +V] extends IterableOnce[(K, V)]:
 def iterator: Iterator[(K, V)]^{this} = ???
final class TreeMap[K, +V]
object TreeMap:
 def fromOrderedEntries[A, B](xs: Iterator[(A, B)]): TreeMap[A, B] = ???
 def from[K, V](it: IterableOnce[(K, V)]^): TreeMap[K, V] =
 it match
 case sm: SortedMap[K, V] =>
 fromOrderedEntries(sm.iterator)
 ???

Bisect pointed to d04f4b5 / #23982
@noti0na1 can you confirm it works as expected before we'll start reverting that for RC2?

I will take a look

The errors look legit to me.

Take immutable.TreeSet.from as an example, the parameter is tracked: it: IterableOnce[E]^. After pattern match, ss should be SortedSet[E]^{it}. Calling iterator will return an Interator^{ss}, which is tracked as well. However, fromOrderedKeys only accepts non-tracked Interator, and causes error.

The correct fix would be updating fromOrderedKeys and fromOrderedEntries.

I don't understand why the errors were not caught during scala-library-bootstrapped on nightly?

The errors should be solved after #24085

Thank you @noti0na1 ! Seems like we don't need to hurry with RC2, @hamzaremmal non bootstrapped tests can be restored as soon as this PR is merged