Fix compatibility of Java with value classes

src/dotty/tools/dotc/core/SymDenotations.scala

@@ -370,6 +370,15 @@ object SymDenotations {
     final def isAnonymousModuleVal(implicit ctx: Context) =
       this.symbol.is(ModuleVal) && (initial.asSymDenotation.name startsWith nme.ANON_CLASS)
 
+    /** Is this a companion class method or companion object method?
+     *  These methods are generated by Symbols#synthesizeCompanionMethod
+     *  and used in SymDenotations#companionClass and
+     *  SymDenotations#companionModule .
+     */
+    final def isCompanionMethod(implicit ctx: Context) =
+      name.toTermName == nme.COMPANION_CLASS_METHOD ||
+      name.toTermName == nme.COMPANION_MODULE_METHOD
+
     /** Is symbol a primitive value class? */
     def isPrimitiveValueClass(implicit ctx: Context) = defn.ScalaValueClasses contains symbol
 

src/dotty/tools/dotc/core/TypeErasure.scala

@@ -142,15 +142,18 @@ object TypeErasure {
    *   - For $asInstanceOf           : [T]T
    *   - For $isInstanceOf           : [T]Boolean
    *   - For all abstract types      : = ?
-   *   - For COMPANION_CLASS_METHOD  : the erasure of their type with semiEraseVCs = false,
-   *                                   this is needed to keep [[SymDenotation#companionClass]]
-   *                                   working after erasure for value classes.
+   *   - For companion methods       : the erasure of their type with semiEraseVCs = false.
+   *                                   The signature of these methods are used to keep a
+   *                                   link between companions and should not be semi-erased.
+   *   - For Java-defined symbols:   : the erasure of their type with isJava = true,
+   *                                   semiEraseVCs = false. Semi-erasure never happens in Java.
    *   - For all other symbols       : the semi-erasure of their types, with
    *                                   isJava, isConstructor set according to symbol.
    */
   def transformInfo(sym: Symbol, tp: Type)(implicit ctx: Context): Type = {
-    val semiEraseVCs = sym.name ne nme.COMPANION_CLASS_METHOD
-    val erase = erasureFn(sym is JavaDefined, semiEraseVCs, sym.isConstructor, wildcardOK = false)
+    val isJava = sym is JavaDefined
+    val semiEraseVCs = !isJava && !sym.isCompanionMethod
+    val erase = erasureFn(isJava, semiEraseVCs, sym.isConstructor, wildcardOK = false)
 
     def eraseParamBounds(tp: PolyType): Type =
       tp.derivedPolyType(

src/dotty/tools/dotc/transform/ValueClasses.scala

@@ -25,7 +25,7 @@ object ValueClasses {
       !d.isConstructor &&
       !d.is(SuperAccessor) &&
       !d.is(Macro) &&
-      !(d.name eq nme.COMPANION_MODULE_METHOD)
+      !d.isCompanionMethod
 
   /** The member that of a derived value class that unboxes it. */
   def valueClassUnbox(d: ClassDenotation)(implicit ctx: Context): Symbol =

tests/pos/valueclasses/t9298/JUse.java

@@ -0,0 +1,7 @@
+package t9298;
+
+class JUse {
+  public static Meter jm() {
+    return new Meter(2);
+  }
+}

tests/pos/valueclasses/t9298/Meter.scala

@@ -0,0 +1,3 @@
+package t9298
+
+class Meter(val x: Int) extends AnyVal

tests/pos/valueclasses/t9298/Use.scala

@@ -0,0 +1,9 @@
+// TODO: this should be a run test once we have run tests
+
+package t9298
+
+object Use {
+  def main(args: Array[String]): Unit = {
+    val x: Meter = JUse.jm
+  }
+}