Emit an error instead of crashing on incorrect Java annotation

This is similar to the error that scalac emits, though it emits at typer
instead of backend.

Author: smarter

compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala

@@ -240,7 +240,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
   private def emitArgument(av:   AnnotationVisitor,
                            name: String,
                            arg:  Tree, bcodeStore: BCodeHelpers)(innerClasesStore: bcodeStore.BCInnerClassGen): Unit = {
-    (normalizeArgument(arg): @unchecked) match {
+    normalizeArgument(arg) match {
       case Literal(const @ Constant(_)) =>
         const.tag match {
           case BooleanTag | ByteTag | ShortTag | CharTag | IntTag | LongTag | FloatTag | DoubleTag => av.visit(name, const.value)
@@ -257,10 +257,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
         av.visit(name, t.args.head.tpe.classSymbol.denot.info.toTypeKind(bcodeStore)(innerClasesStore).toASMType)
       case Ident(nme.WILDCARD) =>
         // An underscore argument indicates that we want to use the default value for this parameter, so do not emit anything
-      case t: tpd.RefTree =>
-        assert(t.symbol.denot.owner.isAllOf(Flags.JavaEnumTrait),
-          i"not an enum: $t / ${t.symbol} / ${t.symbol.denot.owner} / ${t.symbol.denot.owner.isTerm} / ${t.symbol.denot.owner.flagsString}")
-
+      case t: tpd.RefTree if t.symbol.denot.owner.isAllOf(Flags.JavaEnumTrait) =>
         val edesc = innerClasesStore.typeDescriptor(t.tpe.asInstanceOf[bcodeStore.int.Type]) // the class descriptor of the enumeration class.
         val evalue = t.symbol.name.mangledString // value the actual enumeration value.
         av.visitEnum(name, edesc, evalue)
@@ -306,6 +303,9 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
         val desc = innerClasesStore.typeDescriptor(typ.asInstanceOf[bcodeStore.int.Type]) // the class descriptor of the nested annotation class
         val nestedVisitor = av.visitAnnotation(name, desc)
         emitAssocs(nestedVisitor, assocs, bcodeStore)(innerClasesStore)
+
+      case t =>
+        ctx.error(ex"Annotation argument is not a constant", t.sourcePos)
     }
   }
 

tests/neg/java-annot/J.java

@@ -0,0 +1,6 @@
+import java.lang.annotation.*;
+
+@Retention(RetentionPolicy.RUNTIME)
+public @interface J {
+    String value();
+}

tests/neg/java-annot/S.scala

@@ -0,0 +1,8 @@
+object C {
+  val cs: String = "cs"
+}
+
+object S {
+  @J(C.cs) // error: Annotation argument is not a constant
+  def f(): Int = 1
+}