Add final flag to synthetic ImplicitFunctionN traits

Author: bishabosha

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -129,7 +129,9 @@ class Definitions {
           ClassInfo(ScalaPackageClass.thisType, cls, ObjectType :: Nil, decls)
       }
     }
-    newClassSymbol(ScalaPackageClass, name, Trait | NoInits, completer)
+    val flags0 = Trait | NoInits
+    val flags = if (name.isImplicitFunction) flags0 | Final else flags0
+    newClassSymbol(ScalaPackageClass, name, flags, completer)
   }
 
   private def newMethod(cls: ClassSymbol, name: TermName, info: Type, flags: FlagSet = EmptyFlags): TermSymbol =

tests/neg/implicit-funs.scala

@@ -0,0 +1,11 @@
+trait IF1 extends (given Int => Unit) // error
+abstract class IF2 extends (given (Int, String) => Unit) // error
+class IF3 extends (ImplicitFunction3[Int, String, Boolean, Unit]) { // error
+  def apply given (Int, String, Boolean) = ()
+}
+
+trait IFXXL extends (given ( // error
+  Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int) => Unit)
+
+val IFOK: given ( // OK
+  Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int,Int) => Unit = ()