diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index 94872dc1e8d2..31863a8ea600 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -1714,8 +1714,8 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
          *  comparison will instantiate or constrain type variables first.
          */
         def isIncomplete(arg1: Type, arg2: Type): Boolean =
-          val arg1d = arg1.strippedDealias
-          val arg2d = arg2.strippedDealias
+          val arg1d = arg1.stripped
+          val arg2d = arg2.stripped
           (v >= 0) && (arg1d.isInstanceOf[AndType] || arg2d.isInstanceOf[OrType])
           ||
           (v <= 0) && (arg1d.isInstanceOf[OrType] || arg2d.isInstanceOf[AndType])
diff --git a/tests/pos/i20078/AbstractShapeBuilder.java b/tests/pos/i20078/AbstractShapeBuilder.java
new file mode 100644
index 000000000000..5ca57c4f70a6
--- /dev/null
+++ b/tests/pos/i20078/AbstractShapeBuilder.java
@@ -0,0 +1,3 @@
+public abstract class AbstractShapeBuilder<B extends AbstractShapeBuilder<B, S>, S extends Shape> {
+	abstract public B addTrait(Trait trait);
+}
\ No newline at end of file
diff --git a/tests/pos/i20078/Shape.java b/tests/pos/i20078/Shape.java
new file mode 100644
index 000000000000..e4b4540362bc
--- /dev/null
+++ b/tests/pos/i20078/Shape.java
@@ -0,0 +1 @@
+public interface Shape {}
diff --git a/tests/pos/i20078/Test.scala b/tests/pos/i20078/Test.scala
new file mode 100644
index 000000000000..b529af7e40c3
--- /dev/null
+++ b/tests/pos/i20078/Test.scala
@@ -0,0 +1,3 @@
+@main def Test =
+  val builder: AbstractShapeBuilder[? <: AbstractShapeBuilder[?, ?], ? <: Shape] = ???
+  List.empty[Trait].foreach(builder.addTrait(_))
\ No newline at end of file
diff --git a/tests/pos/i20078/Trait.java b/tests/pos/i20078/Trait.java
new file mode 100644
index 000000000000..8082056c1e26
--- /dev/null
+++ b/tests/pos/i20078/Trait.java
@@ -0,0 +1 @@
+public interface Trait {}