Scala object extends nothing

Question

I came across a piece of code in our codebase, which looked invalid to me, but compiled successfully and worked.

object Main extends {
  def main(args: Array[String]): Unit = {
    print("Hello World")
  }
}

Hello World

Can someone explain me, what happens here?
Does Main class extend here an anonymous class/trait?

Answer 1

If we decompile the code using scala -Xprint:typer, we see that Main extends AnyRef:

scalac -Xprint:typer Main.scala                                                                                               
[[syntax trees at end of                     typer]] // Main.scala
package com.yuvalitzchakov {
  object Main extends scala.AnyRef {
    def <init>(): com.yuvalitzchakov.Main.type = {
      Main.super.<init>();
      ()
    };
    def main(args: Array[String]): Unit = scala.Predef.print("Hello World")
  }
}

This is also documented in the Scala specification under object/class definition:

An object definition defines a single object of a new class. Its most general form is object m extends t. Here, m is the name of the object to be defined, and t is a template of the form

sc with mt1 with … with mtn { stats } which defines the base classes, behavior and initial state of m. The extends clause extends sc with mt1 with … with mtn can be omitted, in which case extends scala.AnyRef is assumed.

This syntax is also valid for early initializers:

abstract class X {
  val name: String
  val size = name.size
}

class Y extends {
  val name = "class Y"
} with X

Answer 2

It desugars into

object Main extends Object

You can check this by compiling with scalac -print.

Grammar states

ClassTemplateOpt  ::=  ‘extends’ ClassTemplate | [[‘extends’] TemplateBody]

where

TemplateBody      ::=  [nl] ‘{’ [SelfType] TemplateStat {semi TemplateStat} ‘}’

so it does seem to be specified as valid if we examine [[‘extends’] TemplateBody], as far as I understand.