Accommodate quasi-constant annotation value

Author: som-snytt

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

@@ -2,7 +2,7 @@ package dotty.tools
 package dotc
 package core
 
-import Symbols._, Types._, Contexts._, Constants._, Phases.*
+import StdNames.*, Symbols.*, Types.*, Contexts.*, Constants.*, Phases.*
 import ast.tpd, tpd.*
 import util.Spans.Span
 import printing.{Showable, Printer}
@@ -43,6 +43,12 @@ object Annotations {
     def argumentConstantString(i: Int)(using Context): Option[String] =
       for (case Constant(s: String) <- argumentConstant(i)) yield s
 
+    def argumentAdaptedConstantString(i: Int)(using Context): Option[String] =
+      argument(i) match
+        case Some(Literal(Constant(s: String))) => Some(s)
+        case Some(TypeApply(Select(Literal(Constant(s: String)), n), _)) if n == nme.asInstanceOf_ => Some(s)
+        case _ => None
+
     /** The tree evaluaton is in progress. */
     def isEvaluating: Boolean = false
 

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).orElse(annot.argumentAdaptedConstantString(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:1:8 ----------------------------------------------------------------------------------
+1 |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 @@
+val u = Unit // error