[FIR] Avoid instanceof List in transformInplace

compiler/fir/psi2fir/tests/org/jetbrains/kotlin/fir/builder/AbstractRawFirBuilderTestCase.kt

@@ -148,7 +148,7 @@ abstract class AbstractRawFirBuilderTestCase : KtParsingTestCase(
             } else {
                 element.transformChildren(this, Unit)
             }
-            return CompositeTransformResult(element)
+            return CompositeTransformResult.single(element)
         }
     }
 

compiler/fir/tree/visitors/org/jetbrains/kotlin/fir/visitors/CompositeTransformResult.kt

@@ -7,28 +7,42 @@ package org.jetbrains.kotlin.fir.visitors
 
 import org.jetbrains.kotlin.fir.FirElement
 
-class CompositeTransformResult<out T : Any>(val a: Any) {
+sealed class CompositeTransformResult<out T : Any>() {
+
+    class Single<out T : Any>(val _single: T) : CompositeTransformResult<T>()
+
+    class Multiple<out T : Any>(val _list: List<T>) : CompositeTransformResult<T>()
+
 
     companion object {
-        fun <T : Any> empty() = CompositeTransformResult<T>(emptyList<T>())
-        fun <T : Any> single(t: T) = CompositeTransformResult<T>(t)
-        fun <T : Any> many(l: List<T>) = CompositeTransformResult<T>(l)
+        fun <T : Any> empty() = CompositeTransformResult.Multiple<T>(emptyList<T>())
+        fun <T : Any> single(t: T) = CompositeTransformResult.Single(t)
+        fun <T : Any> many(l: List<T>) = CompositeTransformResult.Multiple(l)
+
+
     }
 
 
-    val isSingle get() = a !is List<*>
-    val isEmpty get() = a is List<*> && a.isEmpty()
+    val list: List<T>
+        get() = when (this) {
+            is CompositeTransformResult.Multiple<*> -> _list as List<T>
+            else -> error("!")
+        }
+
 
     val single: T
-        get() {
-            assert(isSingle)
-            return a as T
+        get() = when (this) {
+            is CompositeTransformResult.Single<*> -> _single as T
+            else -> error("!")
         }
 
-    val list: List<T>
-        get() {
-            return a as List<T>
-        }
+    val isSingle
+        get() = this is CompositeTransformResult.Single<*>
+
+    val isEmpty
+        get() = this is CompositeTransformResult.Multiple<*> && this.list.isEmpty()
+
+
 }
 
 @Suppress("NOTHING_TO_INLINE")