[Test] Add regression test for KT-66336

compiler/testData/diagnostics/tests/inference/forks/forkInWhen.kt

@@ -0,0 +1,20 @@
+// FIR_IDENTICAL
+// ISSUE: KT-66336
+
+interface A : Comparable<A>
+interface B : Comparable<B>
+
+open class Base<T>
+class Derived<T : Comparable<T>, in S : T?>(expr: Base<in S>) : Base<T?>()
+
+fun call(f: (String) -> Base<*>? = { null }) {}
+
+fun test(b: Base<B>) {
+    call {
+        when (it) {
+            "a" -> b
+            "b" -> Derived(Base<A>())
+            else -> null
+        }
+    }
+}