Minor. Add more tests

compiler/testData/codegen/box/inlineClasses/toStringOfUnboxedNullableGeneric.kt

@@ -0,0 +1,25 @@
+// WITH_STDLIB
+// IGNORE_BACKEND: JVM
+// IGNORE_LIGHT_ANALYSIS
+// WORKS_WHEN_VALUE_CLASS
+// LANGUAGE: +ValueClasses, +GenericInlineClassParameter
+
+OPTIONAL_JVM_INLINE_ANNOTATION
+value class IC<T: String>(val x: T)
+
+fun <T: String> IC<T>?.foo() = toString() // `IC?` unboxed into `String?`
+fun <T: String> IC<T>?.bar() = "$this"
+
+fun assertEquals(a: String, b: String) {
+    if (a != b) throw AssertionError("$a != $b")
+}
+
+fun box(): String {
+    assertEquals((null as IC<String>?).foo(), "null")
+    assertEquals((null as IC<String>?).foo(), (null as IC<String>?).toString())
+    assertEquals((null as IC<String>?).foo(), (null as IC<String>?).bar())
+    assertEquals(IC("x").foo(), "IC(x=x)")
+    assertEquals(IC("x").foo(), IC("x").toString())
+    assertEquals(IC("x").foo(), IC("x").bar())
+    return "OK"
+}