SIP-27 Tweak and test wildstar in REPL

From scala#5245 (comment)

src/compiler/scala/tools/nsc/ast/parser/Scanners.scala

@@ -392,8 +392,11 @@ trait Scanners extends ScannersCommon {
         // then it is a trailing comma and is ignored
         val saved = new ScannerData {} copyFrom this
         fetchToken()
-        if (token != RPAREN && token != RBRACKET && token != RBRACE || !afterLineEnd())
-          this copyFrom saved
+        if (afterLineEnd() && (token == RPAREN || token == RBRACKET || token == RBRACE)) {
+          /* skip the trailing comma */
+        } else if (token == EOF) { // e.g. when the REPL is parsing "val List(x, y, _*,"
+          /* skip the trailing comma */
+        } else this copyFrom saved
       }
 
 //      print("["+this+"]")

test/files/pos/trailing-commas.scala

@@ -123,6 +123,10 @@ trait SimplePattern {
     case List(1, 2, _ @ _*,
     ) => 1
   }
+
+  // test varargs in patterns
+  val List(x, y, _*,
+  ) = 42 :: 17 :: Nil
 }
 
 trait ImportSelectors {

test/files/run/trailing-commas.check

@@ -0,0 +1,9 @@
+
+scala> // test varargs in patterns
+
+scala> val List(x, y, _*,
+) = 42 :: 17 :: Nil
+x: Int = 42
+y: Int = 17
+
+scala> :quit

test/files/run/trailing-commas.scala

@@ -0,0 +1,7 @@
+object Test extends scala.tools.partest.ReplTest {
+  def code = """
+// test varargs in patterns
+val List(x, y, _*,
+) = 42 :: 17 :: Nil
+"""
+}