Fix crash: ensure class type is applied

Otherwise, looking up members will crash the compiler

Author: liufengyun

tests/init/crash/singleton-ops-test-issue-8287.scala

@@ -0,0 +1,13 @@
+import scala.compiletime.ops.int._
+
+object Test {
+  class Vec[S <: Int] {
+    infix def concat [RS <: Int](that : Vec[RS]) : Vec[S + RS] = new Vec[S + RS]
+  }
+
+  val v1 = new Vec[1]
+  val v2 = new Vec[2]
+  val v3 : Vec[3] = v1 concat v2
+  val v3a = v1 concat v2
+  val v3b : Vec[3] = v3a
+}