Fix compatibility of Java with value classes

This avoids getting a runtime error when calling a Java-defined method
whose signature contains value classes. It happened because we erased
the value classes in this signature even though it comes from a
classfile.

Amusingly, this problem also exists in scalac:
<https://issues.scala-lang.org/browse/SI-9298>

Author: smarter

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

@@ -145,12 +145,15 @@ object TypeErasure {
    *   - 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.isCompanionMethod
-    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(

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
+  }
+}