Improve invariant checks for erased terms

Instead of erasing an erased term to `???` we erase it to `erasedValue[T]`.
This has 2 advantages, first, the term does not lose its type, and second, the
term is still marked as erased. The second implies that if there is a bug in
the compiler or a macro where the term might end outside an erased context, the
code will not compiler. Currently, the code compiles and then throws when
calling the spurious `???`. See scala#11996.

Author: nicolasstucki

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

@@ -243,10 +243,8 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
     private object dropInlines extends TreeMap {
       override def transform(tree: Tree)(using Context): Tree = tree match {
         case Inlined(call, _, expansion) =>
-          val newExpansion = tree.tpe match
-            case ConstantType(c) => Literal(c)
-            case _ => Typed(ref(defn.Predef_undefined), TypeTree(tree.tpe))
-          cpy.Inlined(tree)(call, Nil, newExpansion.withSpan(tree.span))
+          val newExpansion = PruneErasedDefs.trivialErasedTree(tree)
+          cpy.Inlined(tree)(call, Nil, newExpansion)
         case _ => super.transform(tree)
       }
     }
@@ -282,7 +280,8 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
               tpd.cpy.Apply(tree)(
                 tree.fun,
                 tree.args.mapConserve(arg =>
-                  if (methType.isImplicitMethod && arg.span.isSynthetic) ref(defn.Predef_undefined)
+                  if (methType.isImplicitMethod && arg.span.isSynthetic)
+                    PruneErasedDefs.trivialErasedTree(arg)
                   else dropInlines.transform(arg)))
             else
               tree
@@ -414,12 +413,12 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
             //     case x: (_: Tree[?])
         case m @ MatchTypeTree(bounds, selector, cases) =>
           // Analog to the case above for match types
-          def tranformIgnoringBoundsCheck(x: CaseDef): CaseDef =
+          def transformIgnoringBoundsCheck(x: CaseDef): CaseDef =
             withMode(Mode.Pattern)(super.transform(x)).asInstanceOf[CaseDef]
           cpy.MatchTypeTree(tree)(
             super.transform(bounds),
             super.transform(selector),
-            cases.mapConserve(tranformIgnoringBoundsCheck)
+            cases.mapConserve(transformIgnoringBoundsCheck)
           )
         case Block(_, Closure(_, _, tpt)) if ExpandSAMs.needsWrapperClass(tpt.tpe) =>
           superAcc.withInvalidCurrentClass(super.transform(tree))

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

@@ -26,6 +26,7 @@ import ast.tpd
  */
 class PruneErasedDefs extends MiniPhase with SymTransformer { thisTransform =>
   import tpd._
+  import PruneErasedDefs._
 
   override def phaseName: String = PruneErasedDefs.name
 
@@ -57,24 +58,24 @@ class PruneErasedDefs extends MiniPhase with SymTransformer { thisTransform =>
   override def transformValDef(tree: ValDef)(using Context): Tree =
     val sym = tree.symbol
     if tree.symbol.isEffectivelyErased && !tree.rhs.isEmpty then
-      cpy.ValDef(tree)(rhs = trivialErasedTree(tree))
+      cpy.ValDef(tree)(rhs = trivialErasedTree(tree.rhs))
     else if hasUninitializedRHS(tree) then
       cpy.ValDef(tree)(rhs = cpy.Ident(tree.rhs)(nme.WILDCARD).withType(tree.tpt.tpe))
     else
       tree
 
   override def transformDefDef(tree: DefDef)(using Context): Tree =
     if (tree.symbol.isEffectivelyErased && !tree.rhs.isEmpty)
-      cpy.DefDef(tree)(rhs = trivialErasedTree(tree))
+      cpy.DefDef(tree)(rhs = trivialErasedTree(tree.rhs))
     else tree
 
-  private def trivialErasedTree(tree: Tree)(using Context): Tree =
-    tree.tpe.widenTermRefExpr.dealias.normalized match
-      case ConstantType(c) => Literal(c)
-      case _ => ref(defn.Predef_undefined)
-
 }
 
 object PruneErasedDefs {
+  import tpd._
+
   val name: String = "pruneErasedDefs"
+
+  def trivialErasedTree(tree: Tree)(using Context): Tree =
+    ref(defn.Compiletime_erasedValue).appliedToType(tree.tpe).withSpan(tree.span)
 }