K2: add test for KT-57649

compiler/testData/diagnostics/tests/inference/emptyIntersectionTypes/emptyIntersectionOnIf.fir.kt

@@ -0,0 +1,8 @@
+// ISSUE: KT-57649
+
+open class A
+abstract class B {
+    fun test(current: A): A? =
+        // K2 reports empty intersection here due to the smart cast from A to B, where A & B aren't compatible
+        <!INFERRED_TYPE_VARIABLE_INTO_EMPTY_INTERSECTION_WARNING("K; B?, A?; multiple incompatible classes; : B, A")!>if (current === this) current else null<!>
+}

compiler/testData/diagnostics/tests/inference/emptyIntersectionTypes/emptyIntersectionOnIf.kt

@@ -0,0 +1,8 @@
+// ISSUE: KT-57649
+
+open class A
+abstract class B {
+    fun test(current: A): A? =
+        // K2 reports empty intersection here due to the smart cast from A to B, where A & B aren't compatible
+        if (current === this) current else null
+}