Report error about val reassignment via backing field since Kotlin 1.3

Also improve message for current warning

 #KT-16681 Fixed

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

@@ -458,7 +458,7 @@ class ControlFlowInformationProvider private constructor(
                     }
                     else {
                         if (KtPsiUtil.isBackingFieldReference(variableDescriptor)) {
-                            report(Errors.VAL_REASSIGNMENT_VIA_BACKING_FIELD.on(expression, variableDescriptor), ctxt)
+                            reportValReassigned(expression, variableDescriptor, ctxt)
                         }
                         else {
                             report(Errors.VAL_REASSIGNMENT.on(expression, variableDescriptor), ctxt)
@@ -476,6 +476,15 @@ class ControlFlowInformationProvider private constructor(
         return false
     }
 
+    private fun reportValReassigned(expression: KtExpression, variableDescriptor: VariableDescriptor, ctxt: VariableInitContext) {
+        val diagnosticFactory = if (languageVersionSettings.supportsFeature(LanguageFeature.RestrictionOfValReassignmentViaBackingField))
+            Errors.VAL_REASSIGNMENT_VIA_BACKING_FIELD_ERROR
+        else
+            Errors.VAL_REASSIGNMENT_VIA_BACKING_FIELD
+
+        report(diagnosticFactory.on(expression, variableDescriptor), ctxt)
+    }
+
     private fun checkAssignmentBeforeDeclaration(ctxt: VariableInitContext, expression: KtExpression) =
             if (ctxt.enterInitState?.isDeclared == true
                 || ctxt.exitInitState?.isDeclared == true

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

@@ -784,6 +784,7 @@ public interface Errors {
 
     DiagnosticFactory1<KtExpression, DeclarationDescriptor> VAL_REASSIGNMENT = DiagnosticFactory1.create(ERROR);
     DiagnosticFactory1<KtExpression, DeclarationDescriptor> VAL_REASSIGNMENT_VIA_BACKING_FIELD = DiagnosticFactory1.create(WARNING);
+    DiagnosticFactory1<KtExpression, DeclarationDescriptor> VAL_REASSIGNMENT_VIA_BACKING_FIELD_ERROR = DiagnosticFactory1.create(ERROR);
     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

@@ -308,7 +308,8 @@ 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(VAL_REASSIGNMENT_VIA_BACKING_FIELD, "Reassignment of read-only property via backing field is deprecated", NAME);
+        MAP.put(VAL_REASSIGNMENT_VIA_BACKING_FIELD_ERROR, "Reassignment of read-only property 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/FieldReassignmentError.kt

@@ -0,0 +1,7 @@
+// !LANGUAGE: +RestrictionOfValReassignmentViaBackingField
+
+val my: Int = 1
+    get() {
+        <!VAL_REASSIGNMENT_VIA_BACKING_FIELD_ERROR!>field<!>++
+        return field
+    }

compiler/testData/diagnostics/tests/backingField/FieldReassignmentError.txt

@@ -0,0 +1,3 @@
+package
+
+public val my: kotlin.Int = 1

compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -1777,6 +1777,12 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("FieldReassignmentError.kt")
+            public void testFieldReassignmentError() throws Exception {
+                String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/backingField/FieldReassignmentError.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("FieldShadow.kt")
             public void testFieldShadow() throws Exception {
                 String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/backingField/FieldShadow.kt");

compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsUsingJavacTestGenerated.java

@@ -1777,6 +1777,12 @@ public class DiagnosticsUsingJavacTestGenerated extends AbstractDiagnosticsUsing
                 doTest(fileName);
             }
 
+            @TestMetadata("FieldReassignmentError.kt")
+            public void testFieldReassignmentError() throws Exception {
+                String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/backingField/FieldReassignmentError.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("FieldShadow.kt")
             public void testFieldShadow() throws Exception {
                 String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/backingField/FieldShadow.kt");

compiler/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -60,6 +60,8 @@ enum class LanguageFeature(
     LateinitTopLevelProperties(KOTLIN_1_2),
     LateinitLocalVariables(KOTLIN_1_2),
 
+    RestrictionOfValReassignmentViaBackingField(KOTLIN_1_3),
+
     // Experimental features
 
     Coroutines(KOTLIN_1_1, ApiVersion.KOTLIN_1_1, "https://kotlinlang.org/docs/diagnostics/experimental-coroutines", State.ENABLED_WITH_WARNING),