JDK 15: new JDK method means we can't call "isEmpty" on an Array[Char] due to ambiguous implicit conversions and can't compile scala.collection.mutable.StringBuilder

[xuwei-k]

reproduction steps

run sbt compile in current 2.13.x branch
https://github.com/scala/scala/tree/8c86b7d7136839538cca0ff8fca50f59437564c0

problem

scala/scala/src/library/scala/collection/mutable/StringBuilder.scala:41:13: cannot override a concrete member without a third member that's overridden by both (this rule is designed to prevent ``accidental overrides'')
[error] override def isEmpty: Boolean (defined in trait SeqOps)
[error]   with <defaultmethod> def isEmpty(): Boolean (defined in trait CharSequence)
[error] final class StringBuilder(val underlying: java.lang.StringBuilder) extends AbstractSeq[Char]
[error]             ^

java.lang.CharSequence has isEmpty method since JDK 15

openjdk/jdk@113c48f

Welcome to Scala 2.13.3 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_201).
Type in expressions for evaluation. Or try :help.

scala> classOf[scala.collection.mutable.StringBuilder].getInterfaces
val res0: Array[Class[_]] = Array(interface scala.collection.mutable.ReusableBuilder, interface scala.collection.mutable.IndexedSeq, interface java.lang.CharSequence, interface java.io.Serializable)

[som-snytt]

Also 2.12.x:

[error] /Users/.../scala/src/library/scala/collection/mutable/StringBuilder.scala:34:13: overriding method isEmpty in trait CharSequence of type ()Boolean;
[error]  method isEmpty in trait IndexedSeqOptimized of type => Boolean cannot override a concrete member without a third member that's overridden by both (this rule is designed to prevent ``accidental overrides'')
[error] final class StringBuilder(private val underlying: JavaStringBuilder)
[error]             ^
[error] one error found
[error] (library / Compile / compileIncremental) Compilation failed
[error] Total time: 44 s, completed Oct 5, 2020, 4:08:52 AM