Refactor parser code that rejects forbidden wildcards

The new API is required for all use cases in this PR.

Author: Blaisorblade

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

@@ -717,15 +717,7 @@ object Parsers {
     /** Same as [[typ]], but if this results in a wildcard it emits a syntax error and
      *  returns a tree for type `Any` instead.
      */
-    def toplevelTyp(): Tree = {
-      val t = typ()
-      findWildcardType(t) match {
-        case Some(wildcardPos) =>
-          syntaxError(UnboundWildcardType(), wildcardPos)
-          scalaAny
-        case None => t
-      }
-    }
+    def toplevelTyp(): Tree = checkWildcard(typ())
 
     /** Type        ::=  [FunArgMods] FunArgTypes `=>' Type
      *                |  HkTypeParamClause `->' Type
@@ -925,7 +917,7 @@ object Parsers {
           else Nil
         first :: rest
       }
-      def typParser() = if (wildOK) typ() else toplevelTyp()
+      def typParser() = checkWildcard(typ(), wildOK)
       if (namedOK && in.token == IDENTIFIER)
         typParser() match {
           case Ident(name) if in.token == EQUALS =>
@@ -1020,6 +1012,17 @@ object Parsers {
       case _ => None
     }
 
+    def rejectWildcard(t: Tree, report: Position => Unit, fallbackTree: Tree): Tree =
+      findWildcardType(t) match {
+        case Some(wildcardPos) =>
+          report(wildcardPos)
+          fallbackTree
+        case None => t
+      }
+
+    def checkWildcard(t: Tree, wildOK: Boolean = false, fallbackTree: Tree = scalaAny): Tree =
+      if (wildOK) t else rejectWildcard(t, syntaxError(UnboundWildcardType(), _), fallbackTree)
+
 /* ----------- EXPRESSIONS ------------------------------------------------ */
 
     /** EqualsExpr ::= `=' Expr