Use ErasedPhantom instead of BoxedUnit as erased unit type.

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -990,4 +990,10 @@ class Definitions {
   /** If the symbol is of the class scala.Phantom.Any or scala.Phantom.Nothing */
   def isPhantomTerminalClass(sym: Symbol) = (sym eq Phantom_AnyClass) || (sym eq Phantom_NothingClass)
 
+
+  lazy val ErasedPhantomType: TypeRef = ctx.requiredClassRef("dotty.runtime.ErasedPhantom")
+  def ErasedPhantomClass(implicit ctx: Context) = ErasedPhantomType.symbol.asClass
+
+  def ErasedPhantom_UNIT(implicit ctx: Context) = ErasedPhantomClass.linkedClass.requiredValue("UNIT")
+
 }

compiler/src/dotty/tools/dotc/core/PhantomErasure.scala

@@ -20,9 +20,9 @@ import dotty.tools.dotc.core.Types.Type
 object PhantomErasure {
 
   /** Returns the default erased type of a phantom type */
-  def erasedPhantomType(implicit ctx: Context): Type = defn.BoxedUnitType
+  def erasedPhantomType(implicit ctx: Context): Type = defn.ErasedPhantomType
 
   /** Returns the default erased tree for a call to Phantom.assume */
-  def erasedAssume(implicit ctx: Context): Tree = ref(defn.BoxedUnit_UNIT)
+  def erasedAssume(implicit ctx: Context): Tree = ref(defn.ErasedPhantom_UNIT)
 
 }

compiler/src/dotty/tools/dotc/transform/Memoize.scala

@@ -99,17 +99,19 @@ import Decorators._
     val NoFieldNeeded = Lazy | Deferred | JavaDefined | (if (ctx.settings.YnoInline.value) EmptyFlags else Inline)
 
     def isErasableBottomField(cls: Symbol): Boolean = {
-      // TODO: For Scala.js, return false if this field is in a js.Object unless it was a Phantom before erasure.
-      //       Could time travel to detect phantom types or add an annotation before erasure.
-      !field.isVolatile && ((cls eq defn.NothingClass) || (cls eq defn.NullClass) || (cls eq defn.BoxedUnitClass))
+      // TODO: For Scala.js, return false if this field is in a js.Object unless it is an ErasedPhantomClass.
+      !field.isVolatile &&
+      ((cls eq defn.NothingClass) || (cls eq defn.NullClass) || (cls eq defn.BoxedUnitClass) || (cls eq defn.ErasedPhantomClass))
     }
 
     def erasedBottomTree(sym: Symbol) = {
       if (sym eq defn.NothingClass) Throw(Literal(Constant(null)))
       else if (sym eq defn.NullClass) Literal(Constant(null))
+      else if (sym eq defn.BoxedUnitClass) ref(defn.BoxedUnit_UNIT)
+      else if (sym eq defn.ErasedPhantomClass) ref(defn.ErasedPhantom_UNIT)
       else {
-        assert(sym eq defn.BoxedUnitClass)
-        ref(defn.BoxedUnit_UNIT)
+        assert(false) // Not in sync with isErasableBottomField
+        EmptyTree
       }
     }
 

library/src/dotty/runtime/ErasedPhantom.java

@@ -0,0 +1,30 @@
+package dotty.runtime;
+
+
+/** Unit type representing an erased phantom value.
+ *
+ *  Based on implementation of BoxedUnit.
+ */
+public final class ErasedPhantom implements java.io.Serializable {
+    private static final long serialVersionUID = 4116021023472525845L;
+
+    public final static ErasedPhantom UNIT = new ErasedPhantom();
+
+    public final static Class<Void> TYPE = java.lang.Void.TYPE;
+
+    private Object readResolve() { return UNIT; }
+
+    private ErasedPhantom() { }
+
+    public boolean equals(java.lang.Object other) {
+        return this == other;
+    }
+
+    public int hashCode() {
+        return 0;
+    }
+
+    public String toString() {
+        return "(\uD83D\uDC7B )";
+    }
+}