Fix Kotlin nested class code generation

Author: ArsenHD

utbot-framework/src/main/kotlin/org/utbot/framework/codegen/model/visitor/CgKotlinRenderer.kt

@@ -13,6 +13,7 @@ import org.utbot.framework.codegen.model.tree.CgAnonymousFunction
 import org.utbot.framework.codegen.model.tree.CgArrayAnnotationArgument
 import org.utbot.framework.codegen.model.tree.CgArrayElementAccess
 import org.utbot.framework.codegen.model.tree.CgArrayInitializer
+import org.utbot.framework.codegen.model.tree.CgAuxiliaryClass
 import org.utbot.framework.codegen.model.tree.CgComparison
 import org.utbot.framework.codegen.model.tree.CgConstructorCall
 import org.utbot.framework.codegen.model.tree.CgDeclaration
@@ -32,6 +33,7 @@ import org.utbot.framework.codegen.model.tree.CgNotNullAssertion
 import org.utbot.framework.codegen.model.tree.CgParameterDeclaration
 import org.utbot.framework.codegen.model.tree.CgParameterizedTestDataProviderMethod
 import org.utbot.framework.codegen.model.tree.CgRegularClass
+import org.utbot.framework.codegen.model.tree.CgSimpleRegion
 import org.utbot.framework.codegen.model.tree.CgSpread
 import org.utbot.framework.codegen.model.tree.CgStaticsRegion
 import org.utbot.framework.codegen.model.tree.CgSwitchCase
@@ -120,8 +122,32 @@ internal class CgKotlinRenderer(context: CgRendererContext, printer: CgPrinter =
         }
         // render static declaration regions inside a companion object
         println()
+
+        // In Kotlin, we put static declarations in a companion object of the class,
+        // but that **does not** apply to nested classes.
+        // They must be located in the class itself, not its companion object.
+        // That is why here we extract all the auxiliary classes from static regions
+        // to form a separate region specifically for them.
+        // See the docs on CgAuxiliaryClass for details on what they represent.
+        val auxiliaryClassesRegion = element.staticDeclarationRegions
+            .flatMap { it.content }
+            .filterIsInstance<CgAuxiliaryClass>()
+            .let { classes -> CgSimpleRegion("Util classes", classes) }
+
+        if (auxiliaryClassesRegion.content.isNotEmpty()) {
+            auxiliaryClassesRegion.accept(this)
+            println()
+        }
+
+        // Here we update the static regions by removing all the auxiliary classes from them.
+        // The remaining content of regions will be rendered inside a companion object.
+        val updatedStaticRegions = element.staticDeclarationRegions.map { region ->
+            val updatedContent = region.content.filterNot { it is CgAuxiliaryClass }
+            CgStaticsRegion(region.header, updatedContent)
+        }
+
         renderCompanionObject {
-            for ((i, staticsRegion) in element.staticDeclarationRegions.withIndex()) {
+            for ((i, staticsRegion) in updatedStaticRegions.withIndex()) {
                 if (i != 0) println()
 
                 staticsRegion.accept(this)