Reclassify test and improve error message

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -2882,14 +2882,18 @@ object Parsers {
      */
     def pattern3(location: Location): Tree =
       val p = infixPattern()
-      if location.inArgs && followingIsVararg() then
+      if followingIsVararg() then
         val start = in.skipToken()
-        p match
-          case p @ Ident(name) if name.isVarPattern =>
-            Typed(p, atSpan(start) { Ident(tpnme.WILDCARD_STAR) })
-          case _ =>
-            syntaxError(em"`*` must follow pattern variable", start)
-            p
+        if location.inArgs then
+          p match
+            case p @ Ident(name) if name.isVarPattern =>
+              Typed(p, atSpan(start) { Ident(tpnme.WILDCARD_STAR) })
+            case _ =>
+              syntaxError(em"`*` must follow pattern variable", start)
+              p
+        else
+          syntaxError(em"bad use of `*` - sequence pattern not allowed here", start)
+          p
       else p
 
     /**  Pattern2    ::=  [id `@'] Pattern3

tests/neg/i8715.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i8715.scala:2:46 -----------------------------------------------------------------------------------
+2 |def Test = List(42) match { case List(xs @ (ys*)) => xs } // error
+  |                                              ^
+  |                                              bad use of `*` - sequence pattern not allowed here

tests/neg/i8715.scala

@@ -0,0 +1,2 @@
+@main
+def Test = List(42) match { case List(xs @ (ys*)) => xs } // error