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

Author: anatoliykmetyuk

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

@@ -345,7 +345,7 @@ object TypeOps:
       }
 
       // Step 3: Intersect base classes of both sides
-      val commonBaseClasses = tp.mapReduceOr(_.baseClasses)(intersect)
+      val commonBaseClasses = tp.mapReduceOr(_.dealias.baseClasses)(intersect)
       val doms = dominators(commonBaseClasses, Nil)
       def baseTp(cls: ClassSymbol): Type =
         tp.baseType(cls).mapReduceOr(identity)(mergeRefinedOrApplied)

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