Improve orDominator

Merge applied type arguments also for abstract and opaque type constructors

Fixes scala#12264

Author: odersky

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

@@ -1981,7 +1981,6 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
    *  @note  We do not admit singleton types in or-types as lubs.
    */
   def lub(tp1: Type, tp2: Type, canConstrain: Boolean = false): Type = /*>|>*/ trace(s"lub(${tp1.show}, ${tp2.show}, canConstrain=$canConstrain)", subtyping, show = true) /*<|<*/ {
-
     if (tp1 eq tp2) tp1
     else if (!tp1.exists) tp1
     else if (!tp2.exists) tp2

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

@@ -343,7 +343,11 @@ object TypeOps:
 
     tp match {
       case tp: OrType =>
-        approximateOr(tp.tp1, tp.tp2)
+        (tp.tp1.dealias, tp.tp2.dealias) match
+          case (tp1 @ AppliedType(tycon1, args1), tp2 @ AppliedType(tycon2, args2)) if tycon1 eq tycon2 =>
+            mergeRefinedOrApplied(tp1, tp2)
+          case (tp1, tp2) =>
+            approximateOr(tp1, tp2)
       case _ =>
         tp
     }

tests/pos/i12264.scala

@@ -0,0 +1,40 @@
+object test1:
+
+  object Html {
+    final opaque type Tag[+N] = String
+    def apply[N](name: String): Tag[N] = ???
+  }
+
+  object HtmlTags {
+    final def br: Html.Tag[Int] = Html("br")
+    final def p = Html[Long]("p")
+  }
+
+  object Test {
+    type Expect = Html.Tag[Any]
+
+    val x = List[Expect](HtmlTags.br, HtmlTags.p) // ok
+
+    val y = List(HtmlTags.br, HtmlTags.p)
+    y: List[Expect] // was error
+  }
+
+class test2:
+  type Tag[+N]
+  object Html:
+    def apply[N](name: String): Tag[N] = ???
+
+  object HtmlTags {
+    final def br: Tag[Int] = Html("br")
+    final def p = Html[Long]("p")
+  }
+
+  object Test {
+    type Expect = Tag[Any]
+
+    val x = List[Expect](HtmlTags.br, HtmlTags.p) // ok
+
+    val y = List(HtmlTags.br, HtmlTags.p)
+    y: List[Expect] // was error
+  }
+