Overriding `copy` of case class results in unhelpful error messages

[gsps]

case class A(x: Int) {
  def copy(x: Int = x) = A(x)
}

results in merge errors and friends:

-- Error: try/CaseClassCopyMergeError.scala:2:20 -------------------------------
2 |  def copy(x: Int = this.x) = A(x)
  |                    ^^^^^^
  |             cannot merge
  |               method copy: (x: Int): A  and
  |               method copy: (x: Int): A;
  |             they are both defined in class A but have matching signatures
  |               (x: Int): A and
  |               (x: Int): A
  |             when seen as members of A(A.this)
-- Error: try/CaseClassCopyMergeError.scala:1:19 -------------------------------
1 |case class A(x: Int) {
  |                   ^
  |             cannot merge
  |               method copy: (x: Int): A  and
  |               method copy: (x: Int): A;
  |             they are both defined in class A but have matching signatures
  |               (x: Int): A and
  |               (x: Int): A
  |             when seen as members of A(A.this)
-- [E119] Duplicate Symbol Error: try/CaseClassCopyMergeError.scala:2:6 --------
2 |  def copy(x: Int = this.x) = A(x)
  |      ^
  |method copy in class A is already defined as def copy(x: Int): A at line 1.
  |The definitions have matching type signatures after erasure.
-- Error: try/CaseClassCopyMergeError.scala:1:0 --------------------------------
1 |case class A(x: Int) {
  |^
  |two or more overloaded variants of method copy have default arguments

We have a use case for this in inox (where we override copy to also also copy over some metadata), but I see how one would like to enforce some semantics for copy on case classes. In any case, this should produce a more intelligible error message.

[Blaisorblade]

IIRC overriding copy in case classes was explicitly supported, and it disabled the generation of copy.

[allanrenucci]

For reference scala/scala#5730

[adriaanm]

I would recommend looking at https://github.com/scala/scala/pull/5816/files#diff-44437929b3df2683c7ed998c20eb5b03R620 (applyUnapplyMethodCompleter) instead. I made a few mistakes in the earlier version.

[odersky]

Should be fixed by now.