Bound TypeRepr by Matchable in Quotes (#21377)

Closes #21282

Author: hamzaremmal

library/src/scala/quoted/Quotes.scala

@@ -2640,7 +2640,7 @@ trait Quotes { self: runtime.QuoteUnpickler & runtime.QuoteMatching =>
     // ----- Types ----------------------------------------------------
 
     /** A type, type constructors, type bounds or NoPrefix */
-    type TypeRepr
+    type TypeRepr <: Matchable
 
     /** Module object of `type TypeRepr`  */
     val TypeRepr: TypeReprModule

tests/pos/i21282.scala

@@ -0,0 +1,18 @@
+//> using options -Xfatal-warnings -source:future-migration
+
+import scala.quoted.*
+
+private def isUnionCanonicalImpl[U: Type](using Quotes): Expr[Unit] =
+  import quotes.reflect.*
+  val u = TypeRepr.of[U].dealiasKeepOpaques
+
+  def inner[U: Type](s: Set[TypeRepr], tr: TypeRepr): Set[TypeRepr] =
+    tr.dealiasKeepOpaques match
+      case OrType(a, b) =>
+        val ss = inner[U](s, a)
+        inner[U](ss, b)
+      case x if s.contains(x) =>
+        report.errorAndAbort(s"Type ${x.show} multiple times (CHECK ALIASES) in union ${u.show}")
+      case x => s + x
+  inner(Set.empty, u)
+  '{ () }