retypechecking during specialization breaks with a local case object

[scabug]

object test {
  class R[@specialized(Double) C]

  def g[@specialized(Double) C](x: R[C]): Unit = {
    case object S
  }
}

sandbox/test.scala:5: error: S is already defined as (compiler-generated) case class companion object S
    case object S
                ^

Here's what gets into retyped

<specialized> def g$mDc$sp(x: test.R[Double]): Unit = {
  case object S extends Object with Product with Serializable {
    def <init>(): S.type = {
      S.super.<init>();
      ()
    };
    override <synthetic> def productPrefix(): String = "S";
    <synthetic> def productArity(): Int = 0;
    <synthetic> def productElement(x$1: Int): Any = {
      case <synthetic> val x1: Int = x$1;
      case4(){
        matchEnd3(throw new IndexOutOfBoundsException(x$1.toString()))
      };
      matchEnd3(x: Any){
        x
      }
    };
    override <synthetic> def productIterator(): Iterator[Any] = runtime.this.ScalaRunTime.typedProductIterator[Any](S.this);
    <synthetic> def canEqual(x$1: Any): Boolean = x$1.$isInstanceOf[S.type]();
    override <synthetic> def hashCode(): Int = 83;
    override <synthetic> def toString(): String = "S";
    <synthetic> private def readResolve(): Object = S()
  };
  var S$module: S.type = _;
  case <stable> def S(): S.type = {
    S$module = new S.type();
    S$module
  };
  ()
}

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-8122?orig=1
Reporter: @retronym
Affected Versions: 2.10.3
Blocks #5464

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@xeno-by said:
Looks related to #5467

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@retronym said:
This case seems harder, as the retypechecking happens after expansion of modules to an accessor method and a module.

See also: #7928 / scala/scala@8076738

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@VladUreche said:
Reduced more:

$ cat spec.scala 
class C[@specialized T] {
  def get(t: T): T = {
    case object Hi
    t
  }
}

$ scalac spec.scala
spec.scala:3: error: Hi is already defined as (compiler-generated) case class companion object Hi
    case object Hi
                ^
one error found

And it affects miniboxing as well: miniboxing/miniboxing-plugin#184

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@VladUreche said:
Fixed in miniboxing, but I'm sure the fix in the compiler can be significantly cleaner, since you have full control over the namer:
miniboxing/miniboxing-plugin@5aaa707