Normalize when verifying if TypeTestCasts are unchecked

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

@@ -5214,6 +5214,10 @@ object Types extends TypeUtils {
     def thatReducesUsingGadt(tp: Type)(using Context): Boolean = tp.underlyingMatchType match
       case mt: MatchType => mt.reducesUsingGadt
       case _ => false
+
+    object Normalizing:
+      def unapply(tp: Type)(using Context): Option[Type] =
+        Some(tp.tryNormalize).filter(_.exists)
   }
 
   enum MatchTypeCasePattern:

compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala

@@ -135,6 +135,7 @@ object TypeTestsCasts {
     def recur(X: Type, P: Type): String = trace(s"recur(${X.show}, ${P.show})") {
       (X <:< P) ||| P.dealias.match
       case _: SingletonType     => ""
+      case MatchType.Normalizing(tp) => recur(X, tp)
       case _: TypeProxy
       if isAbstract(P)          => i"it refers to an abstract type member or type parameter"
       case defn.ArrayOf(tpT)    =>

tests/warn/i13433c/A_1.scala

@@ -0,0 +1,27 @@
+import scala.reflect.TypeTest
+
+type Matcher[A] = A match { case String => A }
+
+def patternMatch[A](a: Any)(using tt: TypeTest[Any, Matcher[A]]): Option[Matcher[A]] = {
+  // type T = RDF.Triple[Rdf]
+  a match {
+    case res: Matcher[A] => Some(res)
+    case _ => None
+  }
+}
+
+def patternMatchWithAlias[A](a: Any)(using tt: TypeTest[Any, Matcher[A]]): Option[Matcher[A]] = {
+  type T = Matcher[A]
+  a match {
+    case res: T => Some(res)
+    case _ => None
+  }
+}
+
+type S = String
+type MS = Matcher[S]
+
+type S2 = MS
+type MS2 = Matcher[S2]
+
+type Mstuck = Matcher[Nothing]

tests/warn/i13433c/B_2.scala

@@ -0,0 +1,23 @@
+
+@main def main = {
+  println(patternMatch[String]("abc"))
+  println(patternMatchWithAlias[String]("abc"))
+  println(patternMatch[String]("abc")(using (s: Any) => {
+    if s.isInstanceOf[Matcher[String]] then Some[s.type & Matcher[String]](s.asInstanceOf[s.type & Matcher[String]]) else None }))
+  println(patternMatchWithAlias[String]("abc")(using (s: Any) => {
+    if s.isInstanceOf[Matcher[String]] then Some[s.type & Matcher[String]](s.asInstanceOf[s.type & Matcher[String]]) else None }))
+
+  println(patternMatch[String](1))
+  println(patternMatchWithAlias[String](1))
+
+  println(patternMatch[String]("abc")(using (s: Any) => {
+    if s.isInstanceOf[S] then Some[s.type & Matcher[String]](s.asInstanceOf[s.type & Matcher[String]]) else None}))
+  println(patternMatch[String]("abc")(using (s: Any) => {
+    if s.isInstanceOf[MS] then Some[s.type & Matcher[String]](s.asInstanceOf[s.type & Matcher[String]]) else None}))
+  println(patternMatch[String]("abc")(using (s: Any) => {
+    if s.isInstanceOf[S2] then Some[s.type & Matcher[String]](s.asInstanceOf[s.type & Matcher[String]]) else None}))
+  println(patternMatch[String]("abc")(using (s: Any) => {
+    if s.isInstanceOf[MS2] then Some[s.type & Matcher[String]](s.asInstanceOf[s.type & Matcher[String]]) else None}))
+  println(patternMatch[String]("abc")(using (s: Any) => {
+    if s.isInstanceOf[Mstuck] then Some[s.type & Matcher[String]](s.asInstanceOf[s.type & Matcher[String]]) else None})) // warn
+}