fix scala#4030: avoid adding duplicate binders

compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -908,17 +908,21 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
     }
 
     /** Add a `Bind` node for each `bound` symbol in a type application `unapp` */
-    def addBinders(unapp: Tree, bound: List[Symbol]) = unapp match {
-      case TypeApply(fn, args) =>
-        def addBinder(arg: Tree) = arg.tpe.stripTypeVar match {
-          case ref: TypeRef if bound.contains(ref.symbol) =>
-            tpd.Bind(ref.symbol, Ident(ref))
-          case _ =>
-            arg
-        }
-        tpd.cpy.TypeApply(unapp)(fn, args.mapConserve(addBinder))
-      case _ =>
-        unapp
+    def addBinders(unapp: Tree, bound: List[Symbol]) = {
+      var remain = bound.toSet
+      unapp match {
+        case TypeApply(fn, args) =>
+          def addBinder(arg: Tree) = arg.tpe.stripTypeVar match {
+            case ref: TypeRef if remain.contains(ref.symbol) =>
+              remain -= ref.symbol
+              tpd.Bind(ref.symbol, Ident(ref))
+            case _ =>
+              arg
+          }
+          tpd.cpy.TypeApply(unapp)(fn, args.mapConserve(addBinder))
+        case _ =>
+          unapp
+      }
     }
 
     def fromScala2x = unapplyFn.symbol.exists && (unapplyFn.symbol.owner is Scala2x)

tests/pos/i4030.scala

@@ -0,0 +1,22 @@
+sealed trait Root[T]
+case object C1 extends Root[Int]
+case object C2 extends Root[String]
+//case class C3[X, Y]() extends Root[X|Y|(X => X)]
+case class C3[X, Y]() extends Root[(X => X)|(Y => Y)|(X => Y)]
+case class C4[X, Y]() extends Root[(X => X)|(Y => Y)|(X => Y)]
+
+object TestGADT {
+  //type Foo // abstract
+
+  def f[A <: Seq[_], B, Foo >: A => B](v: Root[Foo], u: Root[Foo]) = (v, u) match {
+    //case C1 =>
+    //case C2 =>
+    case (C3(), C3()) =>
+  }
+  //f(C3[Int, Int]())
+  //implicitly[Int <:< Int|(Int => Int)]
+  //implicitly[(Int => Int) <:< String|(Int => Int)]//($conforms[Int => Int])
+  //f[Int, Int, Int|(Int => Int)](C3[Int, Int]())
+  //f(C3[Int, Int]())
+  f(C3[Seq[_], Long](), C4[Seq[_], Long]())
+}