FIR: Load final static constant Java fields as non-nullable

compiler/testData/diagnostics/tests/j+k/types/constantStaticStringField.fir.kt

@@ -0,0 +1,27 @@
+// SKIP_TXT
+
+// FILE: JavaClass.java
+public class JavaClass {
+    public final static String X = "";
+    public final static String Y = "";
+}
+
+// FILE: main.kt
+
+fun nullableStr(): String? = null
+
+fun foo(w: Int): String {
+    var x: String? = nullableStr()
+
+    if (x != null) {
+        return x
+    }
+
+    // Smartcast should work because when's entries' results are non-flexible
+    x = nullableStr() ?: when (w) {
+        1 -> JavaClass.X
+        else -> JavaClass.Y
+    }
+
+    return x
+}

compiler/testData/diagnostics/tests/j+k/types/constantStaticStringField.kt

@@ -0,0 +1,27 @@
+// SKIP_TXT
+
+// FILE: JavaClass.java
+public class JavaClass {
+    public final static String X = "";
+    public final static String Y = "";
+}
+
+// FILE: main.kt
+
+fun nullableStr(): String? = null
+
+fun foo(w: Int): String {
+    var x: String? = nullableStr()
+
+    if (x != null) {
+        return <!DEBUG_INFO_SMARTCAST!>x<!>
+    }
+
+    // Smartcast should work because when's entries' results are non-flexible
+    x = nullableStr() ?: when (w) {
+        1 -> JavaClass.X
+        else -> JavaClass.Y
+    }
+
+    return <!DEBUG_INFO_SMARTCAST!>x<!>
+}