Vararg patterns must be variable patterns or wildcards

Fixes #11457

Author: odersky

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

@@ -2628,13 +2628,19 @@ object Parsers {
         ascription(p, location)
       else p
 
-    /**  Pattern3    ::=  InfixPattern [‘*’]
+    /**  Pattern3    ::=  InfixPattern
+     *                 |  PatVar ‘*’
      */
     def pattern3(): Tree =
       val p = infixPattern()
       if followingIsVararg() then
         atSpan(in.skipToken()) {
-          Typed(p, Ident(tpnme.WILDCARD_STAR))
+          p match
+            case p @ Ident(name) if name.isVarPattern =>
+              Typed(p, Ident(tpnme.WILDCARD_STAR))
+            case _ =>
+              syntaxError(em"`*` must follow pattern variable")
+              p
         }
       else p
 
@@ -2726,7 +2732,7 @@ object Parsers {
       if (in.token == RPAREN) Nil else patterns(location)
 
     /** ArgumentPatterns  ::=  ‘(’ [Patterns] ‘)’
-     *                      |  ‘(’ [Patterns ‘,’] Pattern2 ‘*’ ‘)’
+     *                      |  ‘(’ [Patterns ‘,’] PatVar ‘*’ ‘)’
      */
     def argumentPatterns(): List[Tree] =
       inParens(patternsOpt(Location.InPatternArgs))

docs/docs/internals/syntax.md

@@ -296,7 +296,7 @@ PatVar            ::=  varid
                     |  ‘_’
 Patterns          ::=  Pattern {‘,’ Pattern}
 ArgumentPatterns  ::=  ‘(’ [Patterns] ‘)’                                       Apply(fn, pats)
-                    |  ‘(’ [Patterns ‘,’] Pattern2 ‘*’ ‘)’
+                    |  ‘(’ [Patterns ‘,’] PatVar ‘*’ ‘)’
 ```
 
 ### Type and Value Parameters

docs/docs/reference/syntax.md

@@ -290,7 +290,7 @@ PatVar            ::=  varid
                     |  ‘_’
 Patterns          ::=  Pattern {‘,’ Pattern}
 ArgumentPatterns  ::=  ‘(’ [Patterns] ‘)’
-                    |  ‘(’ [Patterns ‘,’] Pattern2 ‘*’ ‘)’
+                    |  ‘(’ [Patterns ‘,’] PatVar ‘*’ ‘)’
 ```
 
 ### Type and Value Parameters

tests/neg/i11457.scala

@@ -0,0 +1,6 @@
+
+val x = Seq(1, 2) match
+    case Seq(x, y*) => println(y) // prints List(2) which looks correct
+
+val y = Seq(1, 2) match
+  case Seq(x, (y)*) => println(y) // error