default no-args constructor should be generated for classes with all-optional arguments

[scabug]

= problem =

For a lot of java libraries or frameworks, a default no-args constructor for a class is required. Some examples:

• RPC
• Serialization
• Servlets

If you have a class with only optional arguments (like class A(i : Int = 0)), you'd expect it would work in such a situation.

There is also no easy way to access default values for constructors from java.

= analysis =

There is really no idiomatic way to create classes that adhere to such an interface in scala, see: http://stackoverflow.com/questions/4045863/scala-extra-no-arg-constructor-plus-default-constructor-parameters

It is not a very common use case, but it would improve java-scala interop by fixing this.

= enhancement recommendation =

There are two ways to fix this:

• Generate a default constructor by default for every class where each constructor parameter is optional
• If the overhead for such a solution is unacceptable, then a annotation marking which classes should get a default constructor could be used

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-4278?orig=1
Reporter: logophobia

[scabug]

@paulp said:
I think this is a very good idea.

[scabug]

@axel22 said:
We will keep this open for the moment, to be discussed during the future meetings.

[scabug]

@lrytz said:
in respect of language uniformity we decided not to fix this one - since it's a problem of interoperability with frameworks, we think it should not be fixed at the language level.

workarounds: repeat a default, or abstract over one, or put one default int the zero-argument constructor

class C(a: A = aDefault, b: B = C.bDefault) {
  def this() { this(b = C.bDefault) }
}
object C { def bDefault = ... }

// OR

class C(a: A = aDefault, b: B) {
  def this() { this(b = bDefault) }
}