Reject wildcard types in using clauses (#19459)

Fixes #19364

Author: smarter

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

@@ -3314,7 +3314,7 @@ object Parsers {
     /** ContextTypes   ::=  FunArgType {‘,’ FunArgType}
      */
     def contextTypes(paramOwner: ParamOwner, numLeadParams: Int, impliedMods: Modifiers): List[ValDef] =
-      val tps = commaSeparated(funArgType)
+      val tps = commaSeparated(() => paramTypeOf(toplevelTyp))
       var counter = numLeadParams
       def nextIdx = { counter += 1; counter }
       val paramFlags = if paramOwner.isClass then LocalParamAccessor else Param

tests/neg/i19364.scala

@@ -0,0 +1,14 @@
+
+def f(using ?): Unit = {} // error: unbound wildcard type
+class C(using ?) {}       // error: unbound wildcard type
+
+def f2(using => ?): Unit = {} // error: unbound wildcard type
+class C2(using => ?) {}       // error: unbound wildcard type
+
+def f3(using ? *): Unit = {} // error: unbound wildcard type // error
+class C3(using ? *) {}       // error: unbound wildcard type // error
+
+def g(using x: ?): Unit = {} // error: unbound wildcard type
+class D(using x: ?) {}       // error: unbound wildcard type
+
+val h: (?) ?=> Unit = ??? // ok, but useless (it's a function from Nothing)