Properties without accessors are generated as

IrGetBackingField / IrSetBackingField expressions.

compiler/testData/ir/irText/expressions/setFieldWithImplicitCast.kt

@@ -0,0 +1,14 @@
+// FILE: Derived.kt
+// IR_FILE: setFieldWithImplicitCast.txt
+class Derived : Base() {
+    fun setValue(v: Any) {
+        if (v is String) {
+            value = v
+        }
+    }
+}
+
+// FILE: Base.java
+public class Base {
+    public String value;
+}