Can't call scala.StringBuilder#length_= in 2.13.0-M5

[xuwei-k]

https://github.com/scala/scala/blob/v2.13.0-M5/src/library/scala/collection/mutable/StringBuilder.scala#L68

Welcome to Scala 2.13.0-M5 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_181).
Type in expressions for evaluation. Or try :help.

scala> val s = new StringBuilder
s: StringBuilder =

scala> s.length = 42
                ^
       error: reassignment to val

scala> s.length_=(42)

Welcome to Scala 2.12.6 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_181).
Type in expressions for evaluation. Or try :help.

scala> val s = new StringBuilder
s: StringBuilder =

scala> s.length = 42
s.length: Int = 42

🤔

[SethTisue]

what's the underlying cause?

[xuwei-k]

minimal reproduce example:

CharSeqJava.java

package example;

public interface CharSeqJava {
  int length();
}

Main.scala

package example

trait CharSeqScala {
  def length: Int
}

class A extends CharSeqJava {
  override def length: Int = 1
  def length_=(value: Int): Unit = {}
}

class B extends CharSeqScala {
  def length: Int = 1
  def length_=(value: Int): Unit = {}
}

class C {
  def length: Int = 1
  def length_=(value: Int): Unit = {}
}

class D extends CharSeqJava with CharSeqScala {
  def length: Int = 1
  def length_=(value: Int): Unit = {}
}

class E extends CharSeqScala with CharSeqJava {
  def length: Int = 1
  def length_=(value: Int): Unit = {}
}

object Main {
  val a = new A
  val b = new B
  val c = new C
  val d = new D
  val e = new E

  // compile error
  a.length = 1

  // success
  b.length = 1

  // success
  c.length = 1

  // success
  d.length = 1

  // compile error
  e.length = 1 
}

[xuwei-k]

Should we swap mixin order? 🤔

• https://github.com/scala/scala/blob/v2.12.6/src/library/scala/collection/mutable/StringBuilder.scala#L31-L37
• https://github.com/scala/scala/blob/v2.13.0-M5/src/library/scala/collection/mutable/StringBuilder.scala#L30-L34

 final class StringBuilder(val underlying: java.lang.StringBuilder) extends AbstractSeq[Char]
+  with java.lang.CharSequence 
   with Builder[Char, String]
   with IndexedSeq[Char]
-  with IndexedSeqOps[Char, IndexedSeq, StringBuilder]
+  with IndexedSeqOps[Char, IndexedSeq, StringBuilder] {
-  with java.lang.CharSequence {

[Jasper-M]

I think we should. Swapping mixin order doesn't seem to be enough...
And emitting a better error for things like this would also be nice.

scala> object foo {
     |   def bar() = 4
     |   def bar_=(a: Int) = println(s"assigning $a")
     | }
defined object foo

scala> foo.bar = 3
<console>:12: error: reassignment to val
       foo.bar = 3
               ^

[adriaanm]

This goes back to scala/scala@b739c4a -- since we're "overriding" a Java method with an empty argument list, we add one to the length method, which then disqualifies it from being a getter (treeInfo.mayBeVarGetter expects a NullaryMethodType, which has no argument lists)