From ef2ffb8c0fcb8fd9dfac937ea002021427567801 Mon Sep 17 00:00:00 2001
From: Som Snytt <som.snytt@gmail.com>
Date: Mon, 15 May 2023 16:23:27 -0700
Subject: [PATCH] Accommodate quasi-constant annotation value

---
 compiler/src/dotty/tools/dotc/core/Annotations.scala  | 8 +++++++-
 compiler/src/dotty/tools/dotc/transform/Erasure.scala | 8 ++++----
 tests/neg/no-unit.check                               | 4 ++++
 tests/neg/no-unit.scala                               | 1 +
 4 files changed, 16 insertions(+), 5 deletions(-)
 create mode 100644 tests/neg/no-unit.check
 create mode 100644 tests/neg/no-unit.scala

diff --git a/compiler/src/dotty/tools/dotc/core/Annotations.scala b/compiler/src/dotty/tools/dotc/core/Annotations.scala
index 1615679a036e..d075d42e619f 100644
--- a/compiler/src/dotty/tools/dotc/core/Annotations.scala
+++ b/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)), nme.asInstanceOf_), _)) => Some(s)
+      case _ => None
+
     /** The tree evaluation is in progress. */
     def isEvaluating: Boolean = false
 
diff --git a/compiler/src/dotty/tools/dotc/transform/Erasure.scala b/compiler/src/dotty/tools/dotc/transform/Erasure.scala
index 25239aee59cf..a265059fd2bd 100644
--- a/compiler/src/dotty/tools/dotc/transform/Erasure.scala
+++ b/compiler/src/dotty/tools/dotc/transform/Erasure.scala
@@ -551,7 +551,7 @@ object Erasure {
     }
 
     /** Check that Java statics and packages can only be used in selections.
-      */
+     */
     private def checkNotErased(tree: Tree)(using Context): tree.type =
       if !ctx.mode.is(Mode.Type) then
         if isErased(tree) then
@@ -565,15 +565,15 @@ 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) =>
+            val message = annot.argumentConstantString(0).orElse(annot.argumentAdaptedConstantString(0)) match
+              case Some(msg) =>
                 val addendum = tree match
                   case tree: RefTree
                   if tree.symbol == defn.Compiletime_deferred && tree.name != nme.deferred =>
                     i".\nNote that `deferred` can only be used under its own name when implementing a given in a trait; `${tree.name}` is not accepted."
                   case _ =>
                     ""
-                (c.stringValue ++ addendum).toMessage
+                (msg + addendum).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."""
diff --git a/tests/neg/no-unit.check b/tests/neg/no-unit.check
new file mode 100644
index 000000000000..faee70accb2a
--- /dev/null
+++ b/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
diff --git a/tests/neg/no-unit.scala b/tests/neg/no-unit.scala
new file mode 100644
index 000000000000..2cb5c84e5b7a
--- /dev/null
+++ b/tests/neg/no-unit.scala
@@ -0,0 +1 @@
+val u = Unit // error