Report inner @varargs methods as errors

Better than a crash :)

Author: TheElectronWill

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

@@ -261,7 +261,12 @@ class ElimRepeated extends MiniPhase with InfoTransformer { thisPhase =>
    *  forwards it to the original method.
    */
   private def addVarArgsForwarder(original: Symbol, isBridge: Boolean)(using Context): Unit =
-    val classInfo = original.owner.info
+    val owner = original.owner
+    if !owner.isClass then
+      report.error("inner methods cannot be annotated with @varargs", original.sourcePos)
+      return
+
+    val classInfo = owner.info
     val decls = classInfo.decls.cloneScope
 
     // For simplicity we always set the varargs flag,

tests/neg/varargs-annot.scala

@@ -25,4 +25,10 @@ object Test {
   @varargs def v6: Int = 1 // error
   @varargs def v7(i: Int*)() = i.sum // error
 
+  def f() =
+    @varargs def inner(s: String*) = () // error
+    inner("wrong")
+
 }
+
+@varargs def topLevel(s: String*) = () // ok