Recursion depth in CommonSupertypes is bounded by maxDepth + 3

compiler/testData/diagnostics/tests/enum/commonSupertype.kt

@@ -0,0 +1,6 @@
+// KT-5943 StackOverflowError from commonSupertype of two enums
+
+enum class A { A }
+enum class B { B }
+
+val x = if (true) A.A else B.B

compiler/testData/diagnostics/tests/enum/commonSupertype.txt

@@ -0,0 +1,65 @@
+package
+
+internal val x: kotlin.Enum<out kotlin.Enum<out kotlin.Enum<out kotlin.Enum<out kotlin.Enum<out kotlin.Any?>>>>>
+
+internal final enum class A : kotlin.Enum<A> {
+    private constructor A()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+    public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    public enum entry A : A {
+        private constructor A()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+        public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+        public class object <class-object-for-A> : A.A {
+            private constructor <class-object-for-A>()
+            public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+            public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+            public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+            public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+            public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+        }
+    }
+
+    // Static members
+    public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): A
+    public final /*synthesized*/ fun values(): kotlin.Array<A>
+}
+
+internal final enum class B : kotlin.Enum<B> {
+    private constructor B()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+    public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    public enum entry B : B {
+        private constructor B()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+        public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+        public class object <class-object-for-B> : B.B {
+            private constructor <class-object-for-B>()
+            public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+            public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+            public final override /*1*/ /*fake_override*/ fun name(): kotlin.String
+            public final override /*1*/ /*fake_override*/ fun ordinal(): kotlin.Int
+            public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+        }
+    }
+
+    // Static members
+    public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): B
+    public final /*synthesized*/ fun values(): kotlin.Array<B>
+}

compiler/testData/diagnostics/tests/generics/recursive.kt

@@ -0,0 +1,11 @@
+// !DIAGNOSTICS: -ASSIGNED_BUT_NEVER_ACCESSED_VARIABLE -UNUSED_VALUE
+
+trait Rec<T>
+class A : Rec<A>
+class B : Rec<B>
+
+fun test(a: A, b: B, c: Boolean) {
+    var ab = if (c) a else b
+    ab = a
+    ab = b
+}

compiler/testData/diagnostics/tests/platformTypes/commonSupertype/recursiveGeneric.kt

@@ -0,0 +1,35 @@
+// !DIAGNOSTICS: -ASSIGNED_BUT_NEVER_ACCESSED_VARIABLE -UNUSED_VALUE
+
+// FILE: p/Rec.java
+
+package p;
+
+public interface Rec<T> {
+
+}
+
+// FILE: p/A.java
+
+package p;
+
+public interface A extends Rec<A> {
+
+}
+
+// FILE: p/B.java
+
+package p;
+
+public interface B extends Rec<B> {
+
+}
+
+// FILE: k.kt
+
+import p.*
+
+fun test(a: A, b: B, c: Boolean) {
+    var ab = if (c) a else b
+    ab = a
+    ab = b
+}

compiler/testData/type-checker-test.kt

@@ -40,4 +40,8 @@ fun f<T>(a : Float) : T {a}
 
 trait Parent
 trait A: Parent
-trait B: Parent
+trait B: Parent
+
+trait Rec<T>
+class ARec : Rec<ARec>
+class BRec : Rec<BRec>