FIR checker: report VAL_REASSIGNMENT_VIA_BACKING_FIELD(_ERROR)

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirOldFrontendDiagnosticsTestGenerated.java

@@ -2091,6 +2091,12 @@ public class FirOldFrontendDiagnosticsTestGenerated extends AbstractFirDiagnosti
                 runTest("compiler/testData/diagnostics/tests/backingField/kt782packageLevel.kt");
             }
 
+            @Test
+            @TestMetadata("LocalDeclarations.kt")
+            public void testLocalDeclarations() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/backingField/LocalDeclarations.kt");
+            }
+
             @Test
             @TestMetadata("SetterWithExplicitType.kt")
             public void testSetterWithExplicitType() throws Exception {

compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt

@@ -427,6 +427,12 @@ object DIAGNOSTICS_LIST : DiagnosticList() {
         val VAL_REASSIGNMENT by error<FirSourceElement, KtExpression> {
             parameter<FirPropertySymbol>("variable")
         }
+        val VAL_REASSIGNMENT_VIA_BACKING_FIELD by warning<FirSourceElement, KtExpression> {
+            parameter<FirPropertySymbol>("variable")
+        }
+        val VAL_REASSIGNMENT_VIA_BACKING_FIELD_ERROR by error<FirSourceElement, KtExpression> {
+            parameter<FirPropertySymbol>("variable")
+        }
         val WRONG_INVOCATION_KIND by warning<FirSourceElement, PsiElement> {
             parameter<AbstractFirBasedSymbol<*>>("declaration")
             parameter<EventOccurrencesRange>("requiredRange")

compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt

@@ -275,6 +275,8 @@ object FirErrors {
     // Control flow diagnostics
     val UNINITIALIZED_VARIABLE by error1<FirSourceElement, KtSimpleNameExpression, FirPropertySymbol>()
     val VAL_REASSIGNMENT by error1<FirSourceElement, KtExpression, FirPropertySymbol>()
+    val VAL_REASSIGNMENT_VIA_BACKING_FIELD by warning1<FirSourceElement, KtExpression, FirPropertySymbol>()
+    val VAL_REASSIGNMENT_VIA_BACKING_FIELD_ERROR by error1<FirSourceElement, KtExpression, FirPropertySymbol>()
     val WRONG_INVOCATION_KIND by warning3<FirSourceElement, PsiElement, AbstractFirBasedSymbol<*>, EventOccurrencesRange, EventOccurrencesRange>()
     val LEAKED_IN_PLACE_LAMBDA by error1<FirSourceElement, PsiElement, AbstractFirBasedSymbol<*>>()
     val WRONG_IMPLIES_CONDITION by warning0<FirSourceElement, PsiElement>()

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirDeclarationCheckerUtils.kt

@@ -5,9 +5,11 @@
 
 package org.jetbrains.kotlin.fir.analysis.checkers.declaration
 
+import org.jetbrains.kotlin.config.LanguageFeature
 import org.jetbrains.kotlin.descriptors.ClassKind
 import org.jetbrains.kotlin.descriptors.Modality
 import org.jetbrains.kotlin.descriptors.Visibilities
+import org.jetbrains.kotlin.fir.FirElement
 import org.jetbrains.kotlin.fir.FirSourceElement
 import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
 import org.jetbrains.kotlin.fir.analysis.checkers.modality
@@ -16,7 +18,11 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors
 import org.jetbrains.kotlin.fir.analysis.diagnostics.reportOn
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.declarations.impl.FirDefaultPropertyAccessor
+import org.jetbrains.kotlin.fir.expressions.FirVariableAssignment
+import org.jetbrains.kotlin.fir.languageVersionSettings
+import org.jetbrains.kotlin.fir.references.FirBackingFieldReference
 import org.jetbrains.kotlin.fir.types.FirImplicitTypeRef
+import org.jetbrains.kotlin.fir.visitors.FirVisitorVoid
 import org.jetbrains.kotlin.lexer.KtTokens
 
 internal fun isInsideExpectClass(containingClass: FirRegularClass, context: CheckerContext): Boolean {
@@ -168,8 +174,36 @@ internal fun checkPropertyAccessors(
     reporter: DiagnosticReporter,
     context: CheckerContext
 ) {
-    property.setter?.source?.let {
+    if (property.isVal) {
-        if (property.isVal) {
+        if (property.getter != null) {
+            var reassignment: FirBackingFieldReference? = null
+            // TODO: consider replacing this with normal VariableAssignmentChecker and reporting all 'field = ...' in getter (not just 1st)
+            // This way is a bit hacky and does not handle diagnostic suppression normally
+            val visitor = object : FirVisitorVoid() {
+                override fun visitElement(element: FirElement) {
+                    element.acceptChildren(this)
+                }
+
+                override fun visitVariableAssignment(variableAssignment: FirVariableAssignment) {
+                    // Report the first violation (to match FE 1.0 behavior)
+                    if (reassignment != null) return
+
+                    val backingFieldReference = variableAssignment.lValue as? FirBackingFieldReference
+                    if (backingFieldReference?.resolvedSymbol?.fir == property) {
+                        reassignment = backingFieldReference
+                    }
+                }
+            }
+            property.getter?.body?.accept(visitor, null)
+            reassignment?.source?.let {
+                if (context.session.languageVersionSettings.supportsFeature(LanguageFeature.RestrictionOfValReassignmentViaBackingField)) {
+                    reporter.reportOn(it, FirErrors.VAL_REASSIGNMENT_VIA_BACKING_FIELD_ERROR, property.symbol, context)
+                } else {
+                    reporter.reportOn(it, FirErrors.VAL_REASSIGNMENT_VIA_BACKING_FIELD, property.symbol, context)
+                }
+            }
+        }
+        property.setter?.source?.let {
             reporter.reportOn(it, FirErrors.VAL_WITH_SETTER, context)
         }
     }

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt

@@ -204,6 +204,8 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.USELESS_VARARG_ON
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VALUE_CLASS_CANNOT_BE_CLONEABLE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VALUE_PARAMETER_WITH_NO_TYPE_ANNOTATION
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VAL_REASSIGNMENT
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VAL_REASSIGNMENT_VIA_BACKING_FIELD
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VAL_REASSIGNMENT_VIA_BACKING_FIELD_ERROR
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VAL_WITH_SETTER
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VARARG_OUTSIDE_PARENTHESES
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VARIABLE_EXPECTED
@@ -594,6 +596,8 @@ class FirDefaultErrorMessages : DefaultErrorMessages.Extension {
             // Control flow diagnostics
             map.put(UNINITIALIZED_VARIABLE, "{0} must be initialized before access", PROPERTY_NAME)
             map.put(VAL_REASSIGNMENT, "Val cannot be reassigned", PROPERTY_NAME)
+            map.put(VAL_REASSIGNMENT_VIA_BACKING_FIELD, "Reassignment of read-only property via backing field is deprecated", PROPERTY_NAME)
+            map.put(VAL_REASSIGNMENT_VIA_BACKING_FIELD_ERROR, "Reassignment of read-only property via backing field", PROPERTY_NAME)
             map.put(
                 WRONG_INVOCATION_KIND,
                 "{2} wrong invocation kind: given {3} case, but {4} case is possible",

compiler/testData/diagnostics/tests/backingField/FieldReassignment_after.fir.kt

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

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

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !LANGUAGE: +RestrictionOfValReassignmentViaBackingField
 
 val my: Int = 1

compiler/testData/diagnostics/tests/backingField/FieldReassignment_before.fir.kt

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

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

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !LANGUAGE: -RestrictionOfValReassignmentViaBackingField
 
 val my: Int = 1

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

@@ -0,0 +1,35 @@
+// FIR_IDENTICAL
+// !LANGUAGE: +RestrictionOfValReassignmentViaBackingField
+
+class Outer {
+    val i: Int = 1
+        get() {
+            class Inner {
+                var i: Int = 2
+                    get() {
+                        field++
+                        return field
+                    }
+                val j: Int = 3
+                    get() {
+                        <!VAL_REASSIGNMENT_VIA_BACKING_FIELD_ERROR!>field<!> = 42
+                        return field
+                    }
+
+                fun innerMember() {
+                    <!VAL_REASSIGNMENT_VIA_BACKING_FIELD_ERROR!>field<!>++
+                }
+            }
+            return field
+        }
+
+    val j: Int = 4
+        get() {
+            fun local() {
+                <!VAL_REASSIGNMENT_VIA_BACKING_FIELD_ERROR!>field<!>++
+                field++
+            }
+            local()
+            return field
+        }
+}

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

@@ -0,0 +1,10 @@
+package
+
+public final class Outer {
+    public constructor Outer()
+    public final val i: kotlin.Int = 1
+    public final val j: kotlin.Int = 4
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/testData/diagnostics/tests/controlFlowAnalysis/backingFieldInsideGetter_after.fir.kt

@@ -1,23 +0,0 @@
-// !LANGUAGE: +RestrictionOfValReassignmentViaBackingField
-
-package a
-
-import java.util.HashSet
-
-val a: MutableSet<String>? = null
-    get() {
-        if (a == null) {
-            field = HashSet()
-        }
-        return a
-    }
-
-class R {
-    val b: String? = null
-        get() {
-            if (b == null) {
-                field = "b"
-            }
-            return b
-        }
-}

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

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !LANGUAGE: +RestrictionOfValReassignmentViaBackingField
 
 package a

compiler/testData/diagnostics/tests/controlFlowAnalysis/backingFieldInsideGetter_before.fir.kt

@@ -1,23 +0,0 @@
-// !LANGUAGE: -RestrictionOfValReassignmentViaBackingField
-
-package a
-
-import java.util.HashSet
-
-val a: MutableSet<String>? = null
-    get() {
-        if (a == null) {
-            field = HashSet()
-        }
-        return a
-    }
-
-class R {
-    val b: String? = null
-        get() {
-            if (b == null) {
-                field = "b"
-            }
-            return b
-        }
-}

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

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !LANGUAGE: -RestrictionOfValReassignmentViaBackingField
 
 package a

compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java

@@ -2097,6 +2097,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                 runTest("compiler/testData/diagnostics/tests/backingField/kt782packageLevel.kt");
             }
 
+            @Test
+            @TestMetadata("LocalDeclarations.kt")
+            public void testLocalDeclarations() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/backingField/LocalDeclarations.kt");
+            }
+
             @Test
             @TestMetadata("SetterWithExplicitType.kt")
             public void testSetterWithExplicitType() throws Exception {

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt

@@ -1229,6 +1229,20 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.VAL_REASSIGNMENT_VIA_BACKING_FIELD) { firDiagnostic ->
+        ValReassignmentViaBackingFieldImpl(
+            firSymbolBuilder.buildVariableSymbol(firDiagnostic.a.fir as FirProperty),
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
+    add(FirErrors.VAL_REASSIGNMENT_VIA_BACKING_FIELD_ERROR) { firDiagnostic ->
+        ValReassignmentViaBackingFieldErrorImpl(
+            firSymbolBuilder.buildVariableSymbol(firDiagnostic.a.fir as FirProperty),
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
     add(FirErrors.WRONG_INVOCATION_KIND) { firDiagnostic ->
         WrongInvocationKindImpl(
             firSymbolBuilder.buildSymbol(firDiagnostic.a.fir as FirDeclaration),

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -862,6 +862,16 @@ sealed class KtFirDiagnostic<PSI: PsiElement> : KtDiagnosticWithPsi<PSI> {
         abstract val variable: KtVariableSymbol
     }
 
+    abstract class ValReassignmentViaBackingField : KtFirDiagnostic<KtExpression>() {
+        override val diagnosticClass get() = ValReassignmentViaBackingField::class
+        abstract val variable: KtVariableSymbol
+    }
+
+    abstract class ValReassignmentViaBackingFieldError : KtFirDiagnostic<KtExpression>() {
+        override val diagnosticClass get() = ValReassignmentViaBackingFieldError::class
+        abstract val variable: KtVariableSymbol
+    }
+
     abstract class WrongInvocationKind : KtFirDiagnostic<PsiElement>() {
         override val diagnosticClass get() = WrongInvocationKind::class
         abstract val declaration: KtSymbol

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -1403,6 +1403,22 @@ internal class ValReassignmentImpl(
     override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
 }
 
+internal class ValReassignmentViaBackingFieldImpl(
+    override val variable: KtVariableSymbol,
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ValReassignmentViaBackingField(), KtAbstractFirDiagnostic<KtExpression> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
+internal class ValReassignmentViaBackingFieldErrorImpl(
+    override val variable: KtVariableSymbol,
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ValReassignmentViaBackingFieldError(), KtAbstractFirDiagnostic<KtExpression> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
 internal class WrongInvocationKindImpl(
     override val declaration: KtSymbol,
     override val requiredRange: EventOccurrencesRange,