Accommodate quasi-constant annotation

Author: som-snytt

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -136,7 +136,8 @@ trait TreeInfo[T <: Untyped] { self: Trees.Instance[T] =>
 
   /** All term arguments of an application in a single flattened list */
   def allArguments(tree: Tree): List[Tree] = unsplice(tree) match {
-    case Apply(fn, args) => allArguments(fn) ::: args
+    case Apply(fn, args) => allArguments(fn) ::: args.flatMap(allArguments)
+    case TypeApply(Select(fn, n), _) if n == nme.asInstanceOf_ => fn :: Nil
     case TypeApply(fn, _) => allArguments(fn)
     case Block(_, expr) => allArguments(expr)
     case _ => Nil

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

@@ -562,9 +562,8 @@ object Erasure {
           report.error(msg, tree.srcPos)
         tree.symbol.getAnnotation(defn.CompileTimeOnlyAnnot) match
           case Some(annot) =>
-            val message = annot.argumentConstant(0) match
-              case Some(c) =>
-                c.stringValue.toMessage
+            val message = annot.argumentConstantString(0) match
+              case Some(c) => c.toMessage
               case _ =>
                 em"""Reference to ${tree.symbol.showLocated} should not have survived,
                     |it should have been processed and eliminated during expansion of an enclosing macro or term erasure."""

tests/neg/no-unit.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/no-unit.scala:2:8 ----------------------------------------------------------------------------------
+2 |val u = Unit // error
+  |        ^^^^
+  |        `Unit` companion object is not allowed in source; instead, use `()` for the unit value

tests/neg/no-unit.scala

@@ -0,0 +1,2 @@
+
+val u = Unit // error