Scala class singleton with non-default constructor parameters

[aaabramov]

I decided to leave this question here also (maybe it is some kind of defect). Just because it can be lost in StackOverflow.
Link to question: Scala class singleton with non-default constructor parameters

Question:
I have already read about issues with objects inheriting from companion classes.

For example:

• Class constructor parameter with default value causes companion object initializer to fail
• super constructor cannot be passed a self reference unless parameter is declared by-name

But I have a bit different issue (I am not sure that it is a bug)

I have the following code:

class Scala(name: String)

import Scala._

object Scala extends Scala(TEST_NAME){
  val TEST_NAME = "test name"
}

Note, that I am use variable defined in scope of companion object and then pass it to super's constructor.

I am getting the following compilation error:

Scala.scala:5: error: super constructor cannot be passed a self reference 
unless parameter is declared by-name

object Scala extends Scala(TEST_NAME){
                           ^
one error found

Other variants that I have tried:

Call by name:

class Scala(name: => String)

import Scala._

object Scala extends Scala(TEST_NAME){
  val TEST_NAME = "test name"
}

Named argument:

class Scala(name: String)

import Scala._

object Scala extends Scala(name = TEST_NAME){
  val TEST_NAME = "test name"
}

Both of them:

class Scala(name: => String)

import Scala._

object Scala extends Scala(name = TEST_NAME){
  val TEST_NAME = "test name"
}

Some environment details:

• java: java version "1.8.0_144"
• javac: javac 1.8.0_144
• scala: Scala code runner version 2.12.3
• scalac: Scala compiler version 2.12.3
• OS: Darwin ***.local 17.0.0 Darwin Kernel Version 17.0.0: Thu Aug 24 21:48:19 PDT 2017; root:xnu-4570.1.46~2/RELEASE_X86_64 x86_64

[SethTisue]

hi, and thanks for the report.

but, this bug tracker isn't the place for things where you think maybe it might be a bug but you're not really sure. those should stay on Stack Overflow or https://users.scala-lang.org. come here once you're 90% sure it's a bug.

some hints for achieving a minimal yet complete bug report here:

• is it necessary that the object and the class be companions, or can they be unrelated? if the companion relationship isn't necessary for reproducing the issue, remove it
• searching on GitHub (not JIRA, that's just archival now and is no longer updated) shows that there are a number of other open tickets involving the "super constructor cannot be passed a self reference unless parameter is declared by-name". what is the relationship of your report to those tickets? it it possible it's a duplicate of one of them?
• is there anything in the Scala Language Specification that would illuminate whether this is a bug or intended behavior...?

[aaabramov]

Hi @SethTisue! Thank you for pointing me. I am not an experienced Scala community participator.

The most relates thing that I found in similar issues is the following comment:

Michael Schmitz (schmmd) said (edited on Sep 23, 2011 1:09:21 AM UTC):
This also causes the same error.

class Foo(t: Any)
class Bar(x: String)
object Bar extends Foo(Bar.param) {
    val param = "bar"
}

error: super constructor cannot be passed a self reference unless parameter is declared by-name

But I have not understood the resolution from any of the tickets.

My question is: should the error message be changed? It seems that suggestion unless parameter is declared by-name is useless. Correct me if I am wrong.

Thank you!

[SethTisue]

let's discuss in one of the aforementioned forums

[aaabramov]

@SethTisue done. Link: https://users.scala-lang.org/t/issue-scala-class-singleton-with-non-default-constructor-parameters/1742

[aaabramov]

FYI, @SethTisue quote from users.scala-lang.org (@yawaramin13m):

yawaramin13m
Thanks. I tried changing the constructor param to by-name and indeed got the same error message. At this point this can be considered a bug.

[som-snytt]

I was in the area of that code and took a look. It notices the error when looking for aliases of super ctor args.