[FIR generator] Replace AbstractElement with ElemenOrRef

In the FIR generator, the `AbstractElement` class was used to
represent either an element type without type arguments applied
(using the `Element` subclass), or an element type with applied type
arguments (using the `ElementWithArguments` subclass).

Instead, it is more logical to use the `Element` class to always
represent a non-parameterized element type, and for a parameterized
element type use the `ElementRef` class, just like we do
in the IR generator.

generators/tree-generator-common/src/org/jetbrains/kotlin/generators/tree/AbstractElement.kt

@@ -10,20 +10,17 @@ import org.jetbrains.kotlin.generators.tree.printer.generics
 /**
  * A common interface representing a FIR or IR tree element.
  */
-interface AbstractElement<Element : AbstractElement<Element, Field>, Field : AbstractField> : FieldContainer, ImplementationKindOwner,
-    TypeRef /* TODO: Replace with ElementOrRef */ {
+interface AbstractElement<Element, Field> : ElementOrRef<Element, Field>, FieldContainer, ImplementationKindOwner
+        where Element : AbstractElement<Element, Field>,
+              Field : AbstractField {
 
     val name: String
 
     val fields: Set<Field>
 
-    val parents: List<Element>
-
     val params: List<TypeVariable>
 
-    // TODO: Remove this property as soon as we replace org.jetbrains.kotlin.fir.tree.generator.model.ElementWithArguments with
-    // a generic ElementRef class
-    val typeArguments: List<TypeArgument>
+    val parents: List<Element>
 
     val parentsArguments: Map<Element, Map<TypeRef, TypeRef>>
 

generators/tree-generator-common/src/org/jetbrains/kotlin/generators/tree/TypeArgument.kt

@@ -1,28 +0,0 @@
-/*
- * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
- * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
- */
-
-package org.jetbrains.kotlin.generators.tree
-
-sealed class TypeArgument(val name: String) {
-    abstract val upperBounds: List<TypeRef>
-}
-
-class SimpleTypeArgument(name: String, val upperBound: TypeRef?) : TypeArgument(name) {
-    override val upperBounds: List<TypeRef> = listOfNotNull(upperBound)
-
-    override fun toString(): String {
-        var result = name
-        if (upperBound != null) {
-            result += " : ${upperBound.typeWithArguments}"
-        }
-        return result
-    }
-}
-
-class TypeArgumentWithMultipleUpperBounds(name: String, override val upperBounds: List<TypeRef>) : TypeArgument(name) {
-    override fun toString(): String {
-        return name
-    }
-}

generators/tree-generator-common/src/org/jetbrains/kotlin/generators/tree/TypeRef.kt

@@ -78,6 +78,41 @@ class ClassRef<P : TypeParameterRef> private constructor(
     override fun toString() = canonicalName
 }
 
+interface ElementOrRef<Element, Field> : ParametrizedTypeRef<ElementOrRef<Element, Field>, NamedTypeParameterRef>, ClassOrElementRef
+        where Element : AbstractElement<Element, Field>,
+              Field : AbstractField {
+    val element: Element
+}
+
+data class ElementRef<Element : AbstractElement<Element, Field>, Field : AbstractField>(
+    override val element: Element,
+    override val args: Map<NamedTypeParameterRef, TypeRef> = emptyMap(),
+    override val nullable: Boolean = false,
+) : ElementOrRef<Element, Field> {
+    override fun copy(args: Map<NamedTypeParameterRef, TypeRef>) = ElementRef(element, args, nullable)
+    override fun copy(nullable: Boolean) = ElementRef(element, args, nullable)
+
+    override val type: String
+        get() = element.type
+    override val packageName: String?
+        get() = element.packageName
+
+    override fun getTypeWithArguments(notNull: Boolean): String {
+        return element.type + generics
+    }
+
+    override fun toString() = buildString {
+        append(element.name)
+        append("<")
+        append(args)
+        append(">")
+        if (nullable) {
+            append("?")
+        }
+    }
+}
+
+
 sealed interface TypeParameterRef : TypeRef
 
 data class PositionTypeParameterRef(