Confusing error message when overriding def with var in structural refinement returning a user-defined value class

[scabug]

This is fine

class VC(val s: String) extends AnyVal

class AImpl {
 def a : VC = new VC("a")
}

class X {

  def build(v: String) = new AImpl {
    private var _a = new VC(v)    
    override def a : VC = _a
    def a_=(newA: String) { _a = new VC(newA) }
  }
}

But this isn't

class VC(val s: String) extends AnyVal

class AImpl {
 def a : VC = new VC("a")
}

class X {
  def build(v: String) = new AImpl {
    override var a = new VC(v)
  }
}

It fails with

error: Parameter type in structural refinement may not refer to a user-defined value class
  def build(v: String) = new AImpl {

Instead of

 error: method a_= overrides nothing

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-7709?orig=1
Reporter: Bruno Bieth (mustaghattack)
Affected Versions: 2.10.2

[scabug]

Bruno Bieth (mustaghattack) said:
Well actually the equivalent of the structural refinement with a var would be

 def build(v: String) = new AImpl {
    private var _a = new VC(v)    
    override def a : VC = _a
    def a_=(newA: VC) { _a = newA }
  }

Which fails with a better error message:

error: Parameter type in structural refinement may not refer to a user-defined value class
    def a_=(newA: VC) { _a = newA }

At least here we know where the error come from (the focus is on the method rather than the whole class).

I believe this is the desired behavior due to implementation restrictions. That's what def checkMethodStructuralCompatible says in Typers.

I change the title to confusing instead of incorrect.