Fix #15405: Dealias Or type constituents when finding its dominator

Author: anatoliykmetyuk

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

@@ -594,7 +594,7 @@ object Types {
       try
         this match
           case tp: TypeProxy =>
-            tp.underlying.baseClasses
+            tp.superType.baseClasses
           case tp: ClassInfo =>
             tp.cls.classDenot.baseClasses
           case _ => Nil

tests/pos/i15405.scala

@@ -0,0 +1,4 @@
+type Validated[A] = A
+class Foo:
+  def >(x: Int) = true
+def foo(v: Foo|Validated[Foo]): Unit = v > 10