Fix crash: ensure class type is applied

Otherwise, looking up type members will crash

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/init/Checking.scala

@@ -203,7 +203,7 @@ object Checking {
         PromoteThis(pot, source, state.path).toErrors
 
     case warm: Warm =>
-      val classRef = warm.classSymbol.typeRef
+      val classRef = warm.classSymbol.info.asInstanceOf[ClassInfo].appliedRef
 
       // only check small classes
       val methods = classRef.decls.filter(sym => sym.is(Flags.Method))

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
+}