Identify structural trees on Match Type qualifiers (#18765)

Author: dwijnand

Diff for: compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -992,6 +992,8 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
       def hasRefinement(qualtpe: Type): Boolean = qualtpe.dealias match
         case defn.PolyFunctionOf(_) =>
           false
+        case tp: MatchType =>
+          hasRefinement(tp.tryNormalize)
         case RefinedType(parent, rname, rinfo) =>
           rname == tree.name || hasRefinement(parent)
         case tp: TypeProxy =>

Diff for: tests/neg/i17192.5.scala

@@ -0,0 +1,13 @@
+class Ifce[BT <: Boolean]:
+  type RT = BT match
+    case true  => this.type { val v1: Int }
+    case false => this.type
+  def cast: RT = this.asInstanceOf[RT]
+
+class Test:
+  def t1: Unit =
+    val full1 = new Ifce[true]().cast
+    val v1 = full1.v1 // error
+//           ^^^^^
+//           Found:    (full1 : Ifce[(true : Boolean)]#RT)
+//           Required: Selectable | Dynamic

Diff for: tests/pos/i17192.scala

@@ -0,0 +1,11 @@
+class Ifce[BT <: Boolean] extends Selectable:
+  type RT = BT match
+    case true  => this.type { val v1: Int }
+    case false => this.type
+  def cast : RT = this.asInstanceOf[RT]
+  def selectDynamic(key: String): Any = ???
+
+class Test:
+  def t1: Unit =
+    val full = (new Ifce[true]).cast
+    val v1 = full.v1