[FIR] Add a REDUNDANT_EXPLICIT_BACKING_FIELD warning

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

@@ -880,6 +880,7 @@ object DIAGNOSTICS_LIST : DiagnosticList("FirErrors") {
         val EXPLICIT_BACKING_FIELD_IN_INTERFACE by error<KtBackingField>(PositioningStrategy.FIELD_KEYWORD)
         val EXPLICIT_BACKING_FIELD_IN_ABSTRACT_PROPERTY by error<KtBackingField>(PositioningStrategy.FIELD_KEYWORD)
         val EXPLICIT_BACKING_FIELD_IN_EXTENSION by error<KtBackingField>(PositioningStrategy.FIELD_KEYWORD)
+        val REDUNDANT_EXPLICIT_BACKING_FIELD by warning<KtBackingField>(PositioningStrategy.FIELD_KEYWORD)
         val ABSTRACT_PROPERTY_IN_PRIMARY_CONSTRUCTOR_PARAMETERS by error<KtModifierListOwner>(PositioningStrategy.ABSTRACT_MODIFIER)
         val LOCAL_VARIABLE_WITH_TYPE_PARAMETERS_WARNING by warning<KtProperty>(PositioningStrategy.TYPE_PARAMETERS_LIST)
         val LOCAL_VARIABLE_WITH_TYPE_PARAMETERS by error<KtProperty>(PositioningStrategy.TYPE_PARAMETERS_LIST)

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

@@ -487,6 +487,7 @@ object FirErrors {
     val EXPLICIT_BACKING_FIELD_IN_INTERFACE by error0<KtBackingField>(SourceElementPositioningStrategies.FIELD_KEYWORD)
     val EXPLICIT_BACKING_FIELD_IN_ABSTRACT_PROPERTY by error0<KtBackingField>(SourceElementPositioningStrategies.FIELD_KEYWORD)
     val EXPLICIT_BACKING_FIELD_IN_EXTENSION by error0<KtBackingField>(SourceElementPositioningStrategies.FIELD_KEYWORD)
+    val REDUNDANT_EXPLICIT_BACKING_FIELD by warning0<KtBackingField>(SourceElementPositioningStrategies.FIELD_KEYWORD)
     val ABSTRACT_PROPERTY_IN_PRIMARY_CONSTRUCTOR_PARAMETERS by error0<KtModifierListOwner>(SourceElementPositioningStrategies.ABSTRACT_MODIFIER)
     val LOCAL_VARIABLE_WITH_TYPE_PARAMETERS_WARNING by warning0<KtProperty>(SourceElementPositioningStrategies.TYPE_PARAMETERS_LIST)
     val LOCAL_VARIABLE_WITH_TYPE_PARAMETERS by error0<KtProperty>(SourceElementPositioningStrategies.TYPE_PARAMETERS_LIST)

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

@@ -15,6 +15,7 @@ import org.jetbrains.kotlin.fir.declarations.impl.FirDefaultPropertyAccessor
 import org.jetbrains.kotlin.fir.declarations.utils.hasExplicitBackingField
 import org.jetbrains.kotlin.fir.expressions.FirErrorExpression
 import org.jetbrains.kotlin.fir.typeContext
+import org.jetbrains.kotlin.fir.types.coneType
 import org.jetbrains.kotlin.fir.types.isSubtypeOf
 
 object FirPropertyFieldTypeChecker : FirPropertyChecker() {
@@ -25,7 +26,7 @@ object FirPropertyFieldTypeChecker : FirPropertyChecker() {
             return
         }
 
-        val typeCheckerContext = context.session.typeContext.newBaseTypeCheckerContext(
+        val typeCheckerContext = context.session.typeContext.newTypeCheckerState(
             errorTypesEqualToAnything = false,
             stubTypesEqualToAnything = false
         )
@@ -38,6 +39,11 @@ object FirPropertyFieldTypeChecker : FirPropertyChecker() {
             reporter.reportOn(backingField.source, FirErrors.PROPERTY_FIELD_DECLARATION_MISSING_INITIALIZER, context)
         }
 
+        if (backingField.returnTypeRef.coneType == declaration.returnTypeRef.coneType) {
+            reporter.reportOn(backingField.source, FirErrors.REDUNDANT_EXPLICIT_BACKING_FIELD, context)
+            return
+        }
+
         if (!backingField.isSubtypeOf(declaration, typeCheckerContext)) {
             checkAsFieldNotSubtype(declaration, context, reporter)
         }

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

@@ -357,6 +357,7 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.RECURSIVE_TYPEALI
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.REDECLARATION
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.REDUNDANT_ANNOTATION_TARGET
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.REDUNDANT_CALL_OF_CONVERSION_METHOD
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.REDUNDANT_EXPLICIT_BACKING_FIELD
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.REDUNDANT_EXPLICIT_TYPE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.REDUNDANT_INLINE_SUSPEND_FUNCTION_TYPE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.REDUNDANT_MODALITY_MODIFIER
@@ -1302,6 +1303,10 @@ class FirDefaultErrorMessages {
                 EXPLICIT_BACKING_FIELD_IN_EXTENSION,
                 "Extension properties cannot have a backing field"
             )
+            map.put(
+                REDUNDANT_EXPLICIT_BACKING_FIELD,
+                "There's no need in an explicit backing field declaration if it has the same type as the property"
+            )
             map.put(ABSTRACT_PROPERTY_IN_PRIMARY_CONSTRUCTOR_PARAMETERS, "This property cannot be declared abstract")
             map.put(LOCAL_VARIABLE_WITH_TYPE_PARAMETERS_WARNING, "Type parameters for local variables are deprecated")
             map.put(LOCAL_VARIABLE_WITH_TYPE_PARAMETERS, "Local variables are not allowed to have type parameters")

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

@@ -2448,6 +2448,12 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.REDUNDANT_EXPLICIT_BACKING_FIELD) { firDiagnostic ->
+        RedundantExplicitBackingFieldImpl(
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.ABSTRACT_PROPERTY_IN_PRIMARY_CONSTRUCTOR_PARAMETERS) { firDiagnostic ->
         AbstractPropertyInPrimaryConstructorParametersImpl(
             firDiagnostic as FirPsiDiagnostic,

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

@@ -1731,6 +1731,10 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = ExplicitBackingFieldInExtension::class
     }
 
+    abstract class RedundantExplicitBackingField : KtFirDiagnostic<KtBackingField>() {
+        override val diagnosticClass get() = RedundantExplicitBackingField::class
+    }
+
     abstract class AbstractPropertyInPrimaryConstructorParameters : KtFirDiagnostic<KtModifierListOwner>() {
         override val diagnosticClass get() = AbstractPropertyInPrimaryConstructorParameters::class
     }

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

@@ -2825,6 +2825,13 @@ internal class ExplicitBackingFieldInExtensionImpl(
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
+internal class RedundantExplicitBackingFieldImpl(
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.RedundantExplicitBackingField(), KtAbstractFirDiagnostic<KtBackingField> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
 internal class AbstractPropertyInPrimaryConstructorParametersImpl(
     firDiagnostic: FirPsiDiagnostic,
     override val token: ValidityToken,