Warn when ??? is used to implement erased definition

Author: nicolasstucki

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

@@ -16,9 +16,13 @@ import ast.tpd
 /** This phase makes all erased term members of classes private so that they cannot
  * conflict with non-erased members. This is needed so that subsequent phases like
  * ResolveSuper that inspect class members work correctly.
- * The phase also replaces all expressions that appear in an erased context by
- * default values. This is necessary so that subsequent checking phases such
- * as IsInstanceOfChecker don't give false negatives.
+ * The phase also checks that `erased` definitions contain a call to `erasedValue`
+ * on their RHS. Then replaces all expressions that appear in an effectively
+ * erased context by default values. This is necessary so that subsequent checking
+ * phases such as IsInstanceOfChecker don't give false negatives.
+ *
+ * TODO: This does not belong in this phase as it has nothing to do with `erased`
+ * definitions. Move it out to keep logic clean.
  * Finally, the phase replaces `compiletime.uninitialized` on the right hand side
  * of a mutable field definition by `_`. This avoids a "is declared erased, but is
  * in fact used" error in Erasure and communicates to Constructors that the
@@ -69,6 +73,12 @@ class PruneErasedDefs extends MiniPhase with SymTransformer { thisTransform =>
  else tree
 
  private def trivialErasedTree(tree: Tree)(using Context): Tree =
+ tree match
+ case tree: ValOrDefDef if !tree.symbol.is(Inline) && !tree.symbol.is(Synthetic) =>
+ if tree.rhs.symbol == defn.Predef_undefined then
+ report.warning("Implementation of erased definition should be `erasedValue`. May require an import of `scala.comiletime.erasedValue`.", tree.rhs)
+ case _ =>
+
  tree.tpe.widenTermRefExpr.dealias.normalized match
  case ConstantType(c) => Literal(c)
  case _ => ref(defn.Predef_undefined)

library/src/scala/compiletime/package.scala

@@ -16,7 +16,7 @@ package object compiletime {
  * the branches.
  * @syntax markdown
  */
- erased def erasedValue[T]: T = ???
+ erased def erasedValue[T]: T = erasedValue[T]
 
  /** Used as the initializer of a mutable class or object field, like this:
  *