Only recurse for TypeTests checkability with normalizing match types

Author: EugeneFlesselle

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

@@ -135,7 +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 tp if tp.isMatchAlias => recur(X, tp.tryNormalize)
+      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/pos/i13433c/A_1.scala renamed to tests/warn/i13433c/A_1.scala

@@ -23,3 +23,5 @@ type MS = Matcher[S]
 
 type S2 = MS
 type MS2 = Matcher[S2]
+
+type Mstuck = Matcher[Nothing]

tests/pos/i13433c/B_2.scala renamed to tests/warn/i13433c/B_2.scala

@@ -1,4 +1,3 @@
-//> using options -Xfatal-warnings -deprecation -feature
 
 @main def main = {
   println(patternMatch[String]("abc"))
@@ -19,4 +18,6 @@
     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
 }