Warn about val reassignment via backing field

#KT-16681 In Progress

compiler/frontend/src/org/jetbrains/kotlin/cfg/ControlFlowInformationProvider.kt

@@ -34,7 +34,6 @@ import org.jetbrains.kotlin.cfg.pseudocodeTraverser.*
 import org.jetbrains.kotlin.config.LanguageFeature
 import org.jetbrains.kotlin.config.LanguageVersionSettings
 import org.jetbrains.kotlin.descriptors.*
-import org.jetbrains.kotlin.descriptors.impl.referencedProperty
 import org.jetbrains.kotlin.diagnostics.Diagnostic
 import org.jetbrains.kotlin.diagnostics.DiagnosticFactory
 import org.jetbrains.kotlin.diagnostics.Errors
@@ -400,18 +399,6 @@ class ControlFlowInformationProvider private constructor(
             varWithValReassignErrorGenerated: MutableCollection<VariableDescriptor>
     ): Boolean {
         val variableDescriptor = ctxt.variableDescriptor
-        val propertyDescriptor = variableDescriptor?.referencedProperty
-        if (KtPsiUtil.isBackingFieldReference(variableDescriptor) && propertyDescriptor != null) {
-            val accessor = getParentOfType(expression, KtPropertyAccessor::class.java)
-            if (accessor != null) {
-                val accessorDescriptor = trace.get(BindingContext.DECLARATION_TO_DESCRIPTOR, accessor)
-                if (propertyDescriptor.getter === accessorDescriptor) {
-                    //val can be reassigned through backing field inside its own getter
-                    return false
-                }
-            }
-        }
-
         val mayBeInitializedNotHere = ctxt.enterInitState?.mayBeInitialized() ?: false
         val hasBackingField = (variableDescriptor as? PropertyDescriptor)?.let {
             trace.get(BindingContext.BACKING_FIELD_REQUIRED, it)
@@ -469,7 +456,12 @@ class ControlFlowInformationProvider private constructor(
                         }
                     }
                     else {
-                        report(Errors.VAL_REASSIGNMENT.on(expression, variableDescriptor), ctxt)
+                        if (KtPsiUtil.isBackingFieldReference(variableDescriptor)) {
+                            report(Errors.VAL_REASSIGNMENT_VIA_BACKING_FIELD.on(expression, variableDescriptor), ctxt)
+                        }
+                        else {
+                            report(Errors.VAL_REASSIGNMENT.on(expression, variableDescriptor), ctxt)
+                        }
                     }
                 }
                 if (isThisOrNoDispatchReceiver) {

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java

@@ -761,6 +761,7 @@ public interface Errors {
     DiagnosticFactory0<KtLambdaExpression> UNUSED_LAMBDA_EXPRESSION = DiagnosticFactory0.create(WARNING);
 
     DiagnosticFactory1<KtExpression, DeclarationDescriptor> VAL_REASSIGNMENT = DiagnosticFactory1.create(ERROR);
+    DiagnosticFactory1<KtExpression, DeclarationDescriptor> VAL_REASSIGNMENT_VIA_BACKING_FIELD = DiagnosticFactory1.create(WARNING);
     DiagnosticFactory1<KtExpression, DeclarationDescriptor> CAPTURED_VAL_INITIALIZATION = DiagnosticFactory1.create(ERROR);
     DiagnosticFactory1<KtExpression, DeclarationDescriptor> CAPTURED_MEMBER_VAL_INITIALIZATION = DiagnosticFactory1.create(ERROR);
     DiagnosticFactory1<KtExpression, DeclarationDescriptor> SETTER_PROJECTED_OUT = DiagnosticFactory1.create(ERROR);

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java

@@ -299,6 +299,7 @@ public class DefaultErrorMessages {
         MAP.put(UNUSED_LAMBDA_EXPRESSION, "The lambda expression is unused. If you mean a block, you can use 'run { ... }'");
 
         MAP.put(VAL_REASSIGNMENT, "Val cannot be reassigned", NAME);
+        MAP.put(VAL_REASSIGNMENT_VIA_BACKING_FIELD, "Val cannot be reassigned via backing field", NAME);
         MAP.put(CAPTURED_VAL_INITIALIZATION, "Captured values initialization is forbidden due to possible reassignment", NAME);
         MAP.put(CAPTURED_MEMBER_VAL_INITIALIZATION, "Captured member values initialization is forbidden due to possible reassignment", NAME);
         MAP.put(SETTER_PROJECTED_OUT, "Setter for ''{0}'' is removed by type projection", NAME);

compiler/testData/diagnostics/tests/backingField/FieldReassignment.kt

@@ -1,5 +1,5 @@
 val my: Int = 1
     get() {
-        field++
+        <!VAL_REASSIGNMENT_VIA_BACKING_FIELD!>field<!>++
         return field
     }

compiler/testData/diagnostics/tests/controlFlowAnalysis/backingFieldInsideGetter.kt

@@ -5,7 +5,7 @@ import java.util.HashSet
 val a: MutableSet<String>? = null
     get() {
         if (a == null) {
-            field = HashSet()
+            <!VAL_REASSIGNMENT_VIA_BACKING_FIELD!>field<!> = HashSet()
         }
         return a
     }
@@ -14,7 +14,7 @@ class R {
     val b: String? = null
         get() {
             if (b == null) {
-                field = "b"
+                <!VAL_REASSIGNMENT_VIA_BACKING_FIELD!>field<!> = "b"
             }
             return b
         }