Regression test and simplify

Author: som-snytt

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -1002,14 +1002,14 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
   /** Read an annotation argument, which is pickled either
    *  as a Constant or a Tree.
    */
-  protected def readAnnotArg(i: Int)(using Context): untpd.Tree = untpd.TypedSplice(bytes(index(i)) match
+  protected def readAnnotArg(i: Int)(using Context): untpd.Tree = untpd.TypedSplice:
+    bytes(index(i)) match
     case TREE => at(i, () => readTree())
     case _ => at(i, () =>
       readConstant() match
         case c: Constant => Literal(c)
         case tp: TermRef => ref(tp)
     )
-  )
 
   /** Read a ClassfileAnnotArg (argument to a classfile annotation)
    */

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

@@ -560,15 +560,15 @@ 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) 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."""

tests/neg/no-unit.check

@@ -0,0 +1,14 @@
+-- [E190] Potential Issue Warning: tests/neg/no-unit.scala:7:2 ---------------------------------------------------------
+7 |  Unit // error
+  |  ^^^^
+  |  Discarded non-Unit value of type object Unit. Add `: Unit` to discard silently.
+  |
+  | longer explanation available when compiling with `-explain`
+-- 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
+-- Error: tests/neg/no-unit.scala:7:2 ----------------------------------------------------------------------------------
+7 |  Unit // error
+  |  ^^^^
+  |  `Unit` companion object is not allowed in source; instead, use `()` for the unit value

tests/neg/no-unit.scala

@@ -0,0 +1,7 @@
+val u = Unit // error
+
+inline def a(inline f: Unit ?=> Unit): Unit = f(using ())
+
+def s5bug = a:
+  print(2)
+  Unit // error