Erasure: properly erase value classes

There are three ways to erase a value class:
- In most case, it should be semi-erased to an ErasedValueType, which will be
  fully erased to its underlying type in ElimErasedValueType.
  This corresponds to semiEraseVCs = true in TypeErasure.
- In a few cases, it should be erased like a normal class, so far this
  seems to be necessary for:
  * The return type of a constructor
  * The underlying type of a ThisType
  * TypeTree nodes inside New nodes
  * TypeApply nodes
  * Arrays
  In these cases, we set semiEraseVCs = false
- When calling `sigName` it should be erased to its underlying type.

This commit implements all these cases. Note that this breaks most tests
because ElimErasedValueType has not been implemented yet, it is part of
the next commit.

1 files changed, 8 insertions, 2 deletions

diff --git a/src/dotty/tools/dotc/core/SymDenotations.scala b/src/dotty/tools/dotc/core/SymDenotations.scala
index bcd46810e..14be606a1 100644
--- a/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -1447,10 +1447,16 @@ object SymDenotations {
 
       def inCache(tp: Type) = baseTypeRefCache.containsKey(tp)
 
-      /** Can't cache types containing type variables which are uninstantiated
-       *  or whose instances can change, depending on typerstate.
+      /** We cannot cache:
+       *  - type variables which are uninstantiated or whose instances can
+       *    change, depending on typerstate.
+       *  - types where the underlying type is an ErasedValueType, because
+       *    this underlying type will change after ElimErasedValueType,
+       *    and this changes subtyping relations. As a shortcut, we do not
+       *    cache ErasedValueType at all.
        */
       def isCachable(tp: Type): Boolean = tp match {
+        case _: TypeErasure.ErasedValueType => false
         case tp: TypeVar => tp.inst.exists && inCache(tp.inst)
         case tp: TypeProxy => inCache(tp.underlying)
         case tp: AndOrType => inCache(tp.tp1) && inCache(tp.tp2)