Private constructors are now accessed via synthetic constructor with DEFAULT_CONSTRUCTOR_MARKER as an additional argument #KT-6299 Fixed

A set of tests provided. Some external tests fixed accordingly.
Companion object creation changed accordingly.
Derived classes now can use base class with the private constructor.
Refactoring of AccessorForFunctionDescriptor.

compiler/testData/codegen/box/privateConstructors/base.kt

@@ -0,0 +1,9 @@
+// See also KT-6299
+public open class Outer private constructor() {
+    class Inner: Outer()
+}
+
+fun box(): String {
+    val outer = Outer.Inner()
+    return "OK"
+}

compiler/testData/codegen/box/privateConstructors/companion.kt

@@ -0,0 +1,11 @@
+// See also KT-6299
+public open class Outer private constructor() {
+    companion object {
+        fun foo() = Outer()
+    }
+}
+
+fun box(): String {
+    val outer = Outer.foo()
+    return "OK"
+}

compiler/testData/codegen/box/privateConstructors/inline.kt

@@ -0,0 +1,11 @@
+// See also KT-6299
+public open class Outer private constructor() {
+    companion object {
+        inline fun foo() = Outer()
+    }
+}
+
+fun box(): String {
+    val outer = Outer.foo()
+    return "OK"
+}

compiler/testData/codegen/box/privateConstructors/inner.kt

@@ -0,0 +1,15 @@
+// See also KT-6299
+public open class Outer private constructor(val s: String) {
+    inner class Inner: Outer("O") {
+        fun foo(): String {
+            return this.s + this@Outer.s
+        }
+    }
+    class Nested: Outer("K") 
+    fun bar() = Inner()
+}
+
+fun box(): String {
+    val inner = Outer.Nested().bar()
+    return inner.foo()
+}

compiler/testData/codegen/box/privateConstructors/secondary.kt

@@ -0,0 +1,9 @@
+// See also KT-6299
+public open class Outer private constructor(val x: Int) {
+    constructor(): this(42)
+}
+
+fun box(): String {
+    val outer = Outer()
+    return "OK"
+}

compiler/testData/codegen/box/privateConstructors/withArguments.kt

@@ -0,0 +1,13 @@
+// See also KT-6299
+public open class Outer private constructor(val s: String, val f: Boolean = true) {
+    class Inner: Outer("xyz")
+    class Other: Outer("abc", true)
+    class Another: Outer("", false)
+}
+
+fun box(): String {
+    val outer = Outer.Inner()
+    val other = Outer.Other()
+    val another = Outer.Another()
+    return "OK"
+}

compiler/testData/codegen/box/privateConstructors/withDefault.kt

@@ -0,0 +1,11 @@
+// See also KT-6299
+public open class Outer private constructor(val x: Int = 0) {
+    class Inner: Outer()
+    class Other: Outer(42)
+}
+
+fun box(): String {
+    val outer = Outer.Inner()
+    val other = Outer.Other()
+    return "OK"
+}

compiler/testData/codegen/box/privateConstructors/withLinkedClasses.kt

@@ -0,0 +1,12 @@
+// See also KT-6299
+public open class Outer private constructor(val p: Outer?) {
+    object First: Outer(null)
+    class Other(p: Outer = First): Outer(p)
+}
+
+fun box(): String {
+    val second = Outer.Other()
+    val third = Outer.Other(second)
+    val fourth = Outer.Other(third)
+    return "OK"
+}

compiler/testData/codegen/box/privateConstructors/withLinkedObjects.kt

@@ -0,0 +1,13 @@
+// See also KT-6299
+public open class Outer private constructor(val p: Outer?) {
+    object Inner: Outer(null)
+    object Other: Outer(Inner)
+    object Another: Outer(Other)
+}
+
+fun box(): String {
+    val outer = Outer.Inner
+    val other = Outer.Other
+    val another = Outer.Another
+    return "OK"
+}

compiler/testData/codegen/box/privateConstructors/withVarargs.kt

@@ -0,0 +1,13 @@
+// See also KT-6299
+public open class Outer private constructor(val s: String, vararg i: Int) {
+    class Inner: Outer("xyz")
+    class Other: Outer("abc", 1, 2, 3)
+    class Another: Outer("", 42)
+}
+
+fun box(): String {
+    val outer = Outer.Inner()
+    val other = Outer.Other()
+    val another = Outer.Another()
+    return "OK"
+}