Where String#+ comes from?

Tmp.scala

object Tmp {

  "zzz" ++ "kkk" // Fails with -Yno-predef
  // after typer the above is scala.this.Predef.augmentString("zzz").++[Char, String](scala.this.Predef.augmentString("kkk"))(scala.this.Predef.StringCanBuildFrom);

  "aaa"+new Object()  // Doesn't fail ever!
  // after typer the above is "aaa".+(new java.this.lang.Object());
  // Why  didn't `+` desugar to something else?
 
  new Object() + "aaa" // Fails with -Yno-predef
  // after typer the above is scala.this.Predef.any2stringadd[Object](new java.this.lang.Object()).+("aaa");

  // import Predef.{any2stringadd => _, StringAdd => _, augmentString => _, _} 

}

[[syntax trees at end of                   cleanup]] // tmp.scala
package <empty> {
  object Tmp extends Object {
    def <init>(): Tmp.type = {
      Tmp.super.<init>();
      new collection.immutable.StringOps(scala.this.Predef.augmentString("zzz")).++(new collection.immutable.StringOps(scala.this.Predef.augmentString("kkk")), scala.this.Predef.StringCanBuildFrom());
      "aaa".+(new Object());
      Predef$any2stringadd.this.+$extension(scala.this.Predef.any2stringadd(new Object()), "aaa");
      ()
    }
  }
}

See http://www.scala-lang.org/files/archive/spec/2.11/12-the-scala-standard-library.html#class-string:

"Scala's String class is usually derived from the standard String class of the underlying host system (and may be identified with it). For Scala clients the class is taken to support in each case a method

def + (that: Any): String

which concatenates its left operand with the textual representation of its right operand."

@demobox, it doesn't explains how to compiler ends up compiling it into StringBuilder#append.

• https://github.com/scala/scala/blob/v2.11.8/src/reflect/scala/reflect/internal/Definitions.scala#L1061
• https://github.com/scala/scala/blob/v2.11.8/src/compiler/scala/tools/nsc/backend/ScalaPrimitives.scala#L215
• https://github.com/scala/scala/blob/v2.11.8/src/compiler/scala/tools/nsc/backend/jvm/BCodeBodyBuilder.scala#L233
• https://github.com/scala/scala/blob/v2.11.8/src/compiler/scala/tools/nsc/backend/jvm/BCodeIdiomatic.scala#L209-L239

scala/scala#5224

Awesome, cool!