[k2, dfa] Smartcasts are not allowed on FirJavaField from other module

Handle case with FirField properly. Previously the code was returning
STABLE_VALUE in that case.

Note that in fact the changed 'when' branch expects only FirJavaField,
as FirFieldImpl should be handled by FirBackingFieldSymbol-check at
the beginning of the method. However, it is impossible to refer to
FirJavaField directly due to module visibility

Enumerate all cases more carefully via exhaustive 'when',
it's more readable and will prevent potential issues in future in case
more subclasses are added.

^KT-58279 Fixed

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/dfa/VariableStorageImpl.kt

@@ -9,14 +9,9 @@ import org.jetbrains.kotlin.descriptors.Modality
 import org.jetbrains.kotlin.descriptors.Visibilities
 import org.jetbrains.kotlin.fir.FirElement
 import org.jetbrains.kotlin.fir.FirSession
-import org.jetbrains.kotlin.fir.declarations.FirAnonymousObject
-import org.jetbrains.kotlin.fir.declarations.FirProperty
-import org.jetbrains.kotlin.fir.declarations.FirRegularClass
+import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.declarations.impl.FirDefaultPropertyAccessor
-import org.jetbrains.kotlin.fir.declarations.utils.isExpect
-import org.jetbrains.kotlin.fir.declarations.utils.isFinal
-import org.jetbrains.kotlin.fir.declarations.utils.modality
-import org.jetbrains.kotlin.fir.declarations.utils.visibility
+import org.jetbrains.kotlin.fir.declarations.utils.*
 import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.moduleData
 import org.jetbrains.kotlin.fir.originalOrSelf
@@ -170,7 +165,25 @@ class VariableStorageImpl(private val session: FirSession) : VariableStorage() {
         if (this !is FirVariableSymbol<*>) return null
         if (this is FirFieldSymbol && !this.isFinal) return PropertyStability.MUTABLE_PROPERTY
 
-        val property = this.fir as? FirProperty ?: return PropertyStability.STABLE_VALUE
+        val property = when (val variable = this.fir) { // intentionally exhaustive 'when'
+            is FirEnumEntry, is FirErrorProperty, is FirValueParameter -> return PropertyStability.STABLE_VALUE
+
+            // NB: FirJavaField is expected here. FirFieldImpl should've been handled by FirBackingFieldSymbol check above
+            is FirField -> {
+                if (variable.isJava)
+                    return variable.determineStabilityByModule()
+                else
+                    errorWithAttachment("Expected to handle non-Java FirFields via symbol-based checks") {
+                        withFirEntry("fir", variable)
+                    }
+            }
+
+            is FirBackingField -> errorWithAttachment("Expected to handle Backing Field entirely via symbol-based checks") {
+                withFirEntry("fir", variable)
+            }
+
+            is FirProperty -> variable
+        }
 
         return when {
             property.delegate != null -> PropertyStability.DELEGATED_PROPERTY
@@ -192,16 +205,19 @@ class VariableStorageImpl(private val session: FirSession) : VariableStorage() {
                     }
                 }
             }
-            else -> {
-                val propertyModuleData = property.originalOrSelf().moduleData
-                val currentModuleData = session.moduleData
-                when (propertyModuleData) {
-                    currentModuleData,
-                    in currentModuleData.friendDependencies,
-                    in currentModuleData.dependsOnDependencies -> PropertyStability.STABLE_VALUE
-                    else -> PropertyStability.ALIEN_PUBLIC_PROPERTY
-                }
-            }
+            else -> property.determineStabilityByModule()
+        }
+    }
+
+    private fun FirVariable.determineStabilityByModule(): PropertyStability {
+        val propertyModuleData = originalOrSelf().moduleData
+        val currentModuleData = session.moduleData
+        return when (propertyModuleData) {
+            currentModuleData,
+            in currentModuleData.friendDependencies,
+            in currentModuleData.dependsOnDependencies,
+            -> PropertyStability.STABLE_VALUE
+            else -> PropertyStability.ALIEN_PUBLIC_PROPERTY
         }
     }
 }

compiler/testData/diagnostics/tests/smartCasts/variables/staticJavaFieldFromOtherModule.kt

@@ -11,8 +11,8 @@ public class J {
 // MODULE: app(lib)
 fun isCast() {
     if (J.staticFinalJava is String) {
-        J.staticFinalJava.length
-        (J.staticFinalJava <!USELESS_CAST!>as String<!>).length
+        <!SMARTCAST_IMPOSSIBLE!>J.staticFinalJava<!>.length
+        (J.staticFinalJava as String).length
     }
 
     if (J.staticNonFinalJava is String) {
@@ -23,7 +23,7 @@ fun isCast() {
 
 fun asCast() {
     J.staticFinalJava as String
-    J.staticFinalJava.length
+    <!SMARTCAST_IMPOSSIBLE!>J.staticFinalJava<!>.length
 
     J.staticNonFinalJava as String
     <!SMARTCAST_IMPOSSIBLE!>J.staticNonFinalJava<!>.length