fix: val can be reassigned through backing field inside its own getter

compiler/frontend/src/org/jetbrains/jet/lang/cfg/JetFlowInformationProvider.java

@@ -270,12 +270,24 @@ public class JetFlowInformationProvider {
             @NotNull JetExpression expression,
             @NotNull Collection<VariableDescriptor> varWithValReassignErrorGenerated
     ) {
+        VariableDescriptor variableDescriptor = ctxt.variableDescriptor;
+        if (isBackingFieldReference(expression) && variableDescriptor instanceof PropertyDescriptor) {
+            PropertyDescriptor propertyDescriptor = (PropertyDescriptor) variableDescriptor;
+            JetPropertyAccessor accessor = PsiTreeUtil.getParentOfType(expression, JetPropertyAccessor.class);
+            if (accessor != null) {
+                DeclarationDescriptor accessorDescriptor = trace.get(BindingContext.DECLARATION_TO_DESCRIPTOR, accessor);
+                if (propertyDescriptor.getGetter() == accessorDescriptor) {
+                    //val can be reassigned through backing field inside its own getter
+                    return false;
+                }
+            }
+        }
+
         boolean isInitializedNotHere = ctxt.enterInitState.isInitialized;
         if (expression.getParent() instanceof JetProperty && ((JetProperty)expression).getInitializer() != null) {
             isInitializedNotHere = false;
         }
         boolean hasBackingField = true;
-        VariableDescriptor variableDescriptor = ctxt.variableDescriptor;
         if (variableDescriptor instanceof PropertyDescriptor) {
             hasBackingField = trace.get(BindingContext.BACKING_FIELD_REQUIRED, (PropertyDescriptor) variableDescriptor);
         }
@@ -366,8 +378,8 @@ public class JetFlowInformationProvider {
     private boolean checkBackingField(@NotNull VariableContext cxtx, @NotNull JetElement element) {
         VariableDescriptor variableDescriptor = cxtx.variableDescriptor;
         boolean[] error = new boolean[1];
-        if (isBackingFieldReference((JetElement) element.getParent(), cxtx, error, false)) return false; // this expression has been already checked
+        if (isCorrectBackingFieldReference((JetElement) element.getParent(), cxtx, error, false)) return false; // this expression has been already checked
-        if (!isBackingFieldReference(element, cxtx, error, true)) return false;
+        if (!isCorrectBackingFieldReference(element, cxtx, error, true)) return false;
         if (error[0]) return true;
         if (!(variableDescriptor instanceof PropertyDescriptor)) {
             report(Errors.NOT_PROPERTY_BACKING_FIELD.on(element), cxtx);
@@ -398,12 +410,13 @@ public class JetFlowInformationProvider {
         return true;
     }
 
-    private boolean isBackingFieldReference(@Nullable JetElement element, VariableContext ctxt, boolean[] error, boolean reportError) {
+    private boolean isCorrectBackingFieldReference(@Nullable JetElement element, VariableContext ctxt, boolean[] error, boolean reportError) {
         error[0] = false;
-        if (element instanceof JetSimpleNameExpression && ((JetSimpleNameExpression) element).getReferencedNameElementType() == JetTokens.FIELD_IDENTIFIER) {
+        if (isBackingFieldReference(element)) {
             return true;
         }
-        if (element instanceof JetDotQualifiedExpression && isBackingFieldReference(((JetDotQualifiedExpression) element).getSelectorExpression(), ctxt, error, false)) {
+        if (element instanceof JetDotQualifiedExpression && isCorrectBackingFieldReference(
+                ((JetDotQualifiedExpression) element).getSelectorExpression(), ctxt, error, false)) {
             if (((JetDotQualifiedExpression) element).getReceiverExpression() instanceof JetThisExpression) {
                 return true;
             }
@@ -415,6 +428,10 @@ public class JetFlowInformationProvider {
         return false;
     }
 
+    public static boolean isBackingFieldReference(@Nullable JetElement element) {
+        return element instanceof JetSimpleNameExpression && ((JetSimpleNameExpression) element).getReferencedNameElementType() == JetTokens.FIELD_IDENTIFIER;
+    }
+
     private void recordInitializedVariables(@NotNull Pseudocode pseudocode, @NotNull Map<Instruction, Edges<Map<VariableDescriptor,PseudocodeVariablesData.VariableInitState>>> initializersMap) {
         Edges<Map<VariableDescriptor, VariableInitState>> initializers = initializersMap.get(pseudocode.getExitInstruction());
         Set<VariableDescriptor> usedVariables = pseudocodeVariablesData.getUsedVariables(pseudocode);

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

@@ -0,0 +1,22 @@
+package a
+
+import java.util.HashSet
+
+val a: MutableSet<String>? = null
+    get() {
+        if (a == null) {
+            $a = HashSet()
+        }
+        return a
+    }
+
+class R {
+    val b: String? = null
+        get() {
+            if (b == null) {
+                $b = "b"
+            }
+            return b
+        }
+}
+

compiler/tests/org/jetbrains/jet/checkers/JetDiagnosticsTestGenerated.java

@@ -824,6 +824,11 @@ public class JetDiagnosticsTestGenerated extends AbstractDiagnosticsTestWithEage
                 JetTestUtils.assertAllTestsPresentByMetadata(this.getClass(), "org.jetbrains.jet.generators.tests.GenerateTests", new File("compiler/testData/diagnostics/tests/controlFlowAnalysis"), "kt", true);
             }
             
+            @TestMetadata("backingFieldInsideGetter.kt")
+            public void testBackingFieldInsideGetter() throws Exception {
+                doTest("compiler/testData/diagnostics/tests/controlFlowAnalysis/backingFieldInsideGetter.kt");
+            }
+            
             @TestMetadata("checkInnerLocalDeclarations.kt")
             public void testCheckInnerLocalDeclarations() throws Exception {
                 doTest("compiler/testData/diagnostics/tests/controlFlowAnalysis/checkInnerLocalDeclarations.kt");