Minor. Add regression test

#KT-39374 Fixed

compiler/testData/codegen/box/contracts/kt39374.kt

@@ -0,0 +1,34 @@
+// IGNORE_BACKEND: NATIVE
+// WITH_RUNTIME
+// KJS_WITH_FULL_RUNTIME
+// WITH_COROUTINES
+// COMMON_COROUTINES_TEST
+
+import kotlin.contracts.*
+import COROUTINES_PACKAGE.*
+import helpers.*
+
+@ExperimentalContracts
+public fun <T> runBlocking(block: suspend () -> T): T {
+    contract {
+        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
+    }
+    var res: T? = null
+    suspend {
+        res = block()
+    }.startCoroutine(EmptyContinuation)
+    return res!!
+}
+
+sealed class S {
+    class Z : S() {
+        fun f(): String = "OK"
+    }
+}
+
+val z: S = S.Z()
+
+@ExperimentalContracts
+fun box(): String = when (val w = z) {
+    is S.Z -> runBlocking { w.f() }
+}