Update doc

Author: nicolasstucki

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

@@ -7,15 +7,11 @@ import dotty.tools.dotc.core.Types.Type
 
 /** Phantom erasure erases (minimal erasure):
  *
- *  - Parameters/arguments are erased to BoxedUnit. The next step will remove the parameters
+ *  - Parameters/arguments are erased to ErasedPhantom. The next step will remove the parameters
  *    from the method definitions and calls (implemented in branch implement-phantom-types-part-2).
- *  - Definitions of `def`, `val`, `lazy val` and `var` returning a phantom type to return a BoxedUnit. Where fields
- *    with BoxedUnit type are not memoized (see transform/Memoize.scala).
- *  - Calls to Phantom.assume become calls to BoxedUnit. Intended to be optimized away by local optimizations.
- *
- *  BoxedUnit is used as it fits perfectly and homogeneously in all locations where phantoms can be found.
- *  Additionally some of the optimizations that can be performed on phantom types can also be directly implemented
- *  on all boxed units.
+ *  - Definitions of `def`, `val`, `lazy val` and `var` returning a phantom type to return a ErasedPhantom. Where fields
+ *    with ErasedPhantom type are not memoized (see transform/Memoize.scala).
+ *  - Calls to Phantom.assume become calls to ErasedPhantom.UNIT. Intended to be optimized away by local optimizations.
  */
 object PhantomErasure {
 

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

@@ -110,7 +110,7 @@ import Decorators._
       else if (sym eq defn.BoxedUnitClass) ref(defn.BoxedUnit_UNIT)
       else if (sym eq defn.ErasedPhantomClass) ref(defn.ErasedPhantom_UNIT)
       else {
-        assert(false) // Not in sync with isErasableBottomField
+        assert(false, sym + " has no erased bottom tree")
         EmptyTree
       }
     }