[K/N] Implement frontend checkers for usages forward declaration type

Forward declaration type doesn't exist in runtime. This restricts
its possible usages.

^KT-59764

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

@@ -7,12 +7,14 @@ package org.jetbrains.kotlin.fir.checkers.generator.diagnostics
 
 import com.intellij.psi.PsiElement
 import org.jetbrains.kotlin.config.LanguageFeature
+import org.jetbrains.kotlin.diagnostics.Severity
 import org.jetbrains.kotlin.fir.PrivateForInline
 import org.jetbrains.kotlin.fir.checkers.generator.diagnostics.model.DiagnosticList
 import org.jetbrains.kotlin.fir.checkers.generator.diagnostics.model.PositioningStrategy
 import org.jetbrains.kotlin.fir.symbols.FirBasedSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirCallableSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirRegularClassSymbol
+import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.psi.KtDeclaration
 import org.jetbrains.kotlin.psi.KtElement
@@ -65,5 +67,19 @@ object NATIVE_DIAGNOSTICS_LIST : DiagnosticList("FirNativeErrors") {
         val INVALID_OBJC_HIDES_TARGETS by error<KtElement>()
         val INVALID_REFINES_IN_SWIFT_TARGETS by error<KtElement>()
         val SUBTYPE_OF_HIDDEN_FROM_OBJC by error<KtElement>()
+
+        val CANNOT_CHECK_FOR_FORWARD_DECLARATION by error<KtElement>() {
+            parameter<ConeKotlinType>("type")
+        }
+        val UNCHECKED_CAST_TO_FORWARD_DECLARATION by warning<KtElement> {
+            parameter<ConeKotlinType>("sourceType")
+            parameter<ConeKotlinType>("destinationType")
+        }
+        val FORWARD_DECLARATION_AS_REIFIED_TYPE_ARGUMENT by error<KtElement> {
+            parameter<ConeKotlinType>("type")
+        }
+        val FORWARD_DECLARATION_AS_CLASS_LITERAL by error<KtElement> {
+            parameter<ConeKotlinType>("type")
+        }
     }
 }

compiler/fir/checkers/checkers.native/build.gradle.kts

@@ -15,6 +15,7 @@ dependencies {
     implementation(project(":compiler:frontend"))
     implementation(project(":compiler:psi"))
 
+    compileOnly(project(":core:compiler.common.native"))
     compileOnly(commonDependency("org.jetbrains.kotlin:kotlin-reflect")) { isTransitive = false }
     compileOnly(intellijCore())
 }

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

@@ -13,6 +13,7 @@ import org.jetbrains.kotlin.diagnostics.rendering.RootDiagnosticRendererFactory
 import org.jetbrains.kotlin.fir.analysis.diagnostics.*
 import org.jetbrains.kotlin.fir.symbols.FirBasedSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirRegularClassSymbol
+import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.psi.KtDeclaration
 import org.jetbrains.kotlin.psi.KtElement
@@ -47,6 +48,10 @@ object FirNativeErrors {
     val INVALID_OBJC_HIDES_TARGETS by error0<KtElement>()
     val INVALID_REFINES_IN_SWIFT_TARGETS by error0<KtElement>()
     val SUBTYPE_OF_HIDDEN_FROM_OBJC by error0<KtElement>()
+    val CANNOT_CHECK_FOR_FORWARD_DECLARATION by error1<KtElement, ConeKotlinType>()
+    val UNCHECKED_CAST_TO_FORWARD_DECLARATION by warning2<KtElement, ConeKotlinType, ConeKotlinType>()
+    val FORWARD_DECLARATION_AS_REIFIED_TYPE_ARGUMENT by error1<KtElement, ConeKotlinType>()
+    val FORWARD_DECLARATION_AS_CLASS_LITERAL by error1<KtElement, ConeKotlinType>()
 
     init {
         RootDiagnosticRendererFactory.registerFactory(FirNativeErrorsDefaultMessages)

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

@@ -8,9 +8,14 @@ package org.jetbrains.kotlin.fir.analysis.diagnostics.native
 import org.jetbrains.kotlin.diagnostics.KtDiagnosticFactoryToRendererMap
 import org.jetbrains.kotlin.diagnostics.KtDiagnosticRenderers.TO_STRING
 import org.jetbrains.kotlin.diagnostics.rendering.BaseDiagnosticRendererFactory
+import org.jetbrains.kotlin.diagnostics.rendering.Renderers
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirDiagnosticRenderers
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirDiagnosticRenderers.SYMBOL
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirDiagnosticRenderers.SYMBOLS
+import org.jetbrains.kotlin.fir.analysis.diagnostics.native.FirNativeErrors.CANNOT_CHECK_FOR_FORWARD_DECLARATION
 import org.jetbrains.kotlin.fir.analysis.diagnostics.native.FirNativeErrors.EMPTY_OBJC_NAME
+import org.jetbrains.kotlin.fir.analysis.diagnostics.native.FirNativeErrors.FORWARD_DECLARATION_AS_CLASS_LITERAL
+import org.jetbrains.kotlin.fir.analysis.diagnostics.native.FirNativeErrors.FORWARD_DECLARATION_AS_REIFIED_TYPE_ARGUMENT
 import org.jetbrains.kotlin.fir.analysis.diagnostics.native.FirNativeErrors.INAPPLICABLE_EXACT_OBJC_NAME
 import org.jetbrains.kotlin.fir.analysis.diagnostics.native.FirNativeErrors.INAPPLICABLE_OBJC_NAME
 import org.jetbrains.kotlin.fir.analysis.diagnostics.native.FirNativeErrors.INAPPLICABLE_SHARED_IMMUTABLE_PROPERTY
@@ -33,6 +38,7 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.native.FirNativeErrors.REDU
 import org.jetbrains.kotlin.fir.analysis.diagnostics.native.FirNativeErrors.NON_LITERAL_OBJC_NAME_ARG
 import org.jetbrains.kotlin.fir.analysis.diagnostics.native.FirNativeErrors.SUBTYPE_OF_HIDDEN_FROM_OBJC
 import org.jetbrains.kotlin.fir.analysis.diagnostics.native.FirNativeErrors.THROWS_LIST_EMPTY
+import org.jetbrains.kotlin.fir.analysis.diagnostics.native.FirNativeErrors.UNCHECKED_CAST_TO_FORWARD_DECLARATION
 
 object FirNativeErrorsDefaultMessages : BaseDiagnosticRendererFactory() {
     override val MAP = KtDiagnosticFactoryToRendererMap("FIR").also { map ->
@@ -79,5 +85,28 @@ object FirNativeErrorsDefaultMessages : BaseDiagnosticRendererFactory() {
         map.put(MISSING_EXACT_OBJC_NAME, "Exact @ObjCName is required to have an ObjC name")
         map.put(NON_LITERAL_OBJC_NAME_ARG, "@ObjCName accepts only literal string and boolean values")
         map.put(SUBTYPE_OF_HIDDEN_FROM_OBJC, "Only @HiddenFromObjC declaration can be a subtype of @HiddenFromObjC declaration")
+
+
+        map.put(
+            CANNOT_CHECK_FOR_FORWARD_DECLARATION,
+            "Cannot check for forward declaration: ''{0}''",
+            FirDiagnosticRenderers.RENDER_TYPE
+        )
+        map.put(
+            UNCHECKED_CAST_TO_FORWARD_DECLARATION,
+            "Unchecked cast to forward declaration: ''{0}'' to ''{1}''",
+            FirDiagnosticRenderers.RENDER_TYPE,
+            FirDiagnosticRenderers.RENDER_TYPE
+        )
+        map.put(
+            FORWARD_DECLARATION_AS_REIFIED_TYPE_ARGUMENT,
+            "Cannot pass forward declaration ''{0}'' for reified type parameter",
+            FirDiagnosticRenderers.RENDER_TYPE
+        )
+        map.put(
+            FORWARD_DECLARATION_AS_CLASS_LITERAL,
+            "Can't refer to forward declaration ''{0}'' from class literal",
+            FirDiagnosticRenderers.RENDER_TYPE
+        )
     }
 }

compiler/fir/checkers/checkers.native/src/org/jetbrains/kotlin/fir/analysis/native/checkers/FirNativeForwardDeclarationGetClassCallChecker.kt

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+import org.jetbrains.kotlin.diagnostics.DiagnosticReporter
+import org.jetbrains.kotlin.diagnostics.reportOn
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.checkers.expression.FirGetClassCallChecker
+import org.jetbrains.kotlin.fir.analysis.checkers.toRegularClassSymbol
+import org.jetbrains.kotlin.fir.analysis.diagnostics.native.FirNativeErrors
+import org.jetbrains.kotlin.fir.analysis.native.checkers.forwardDeclarationKindOrNull
+import org.jetbrains.kotlin.fir.expressions.*
+import org.jetbrains.kotlin.fir.types.coneType
+
+
+object FirNativeForwardDeclarationGetClassCallChecker : FirGetClassCallChecker() {
+    override fun check(expression: FirGetClassCall, context: CheckerContext, reporter: DiagnosticReporter) {
+        val declarationToCheck = expression.argument.typeRef.toRegularClassSymbol(context.session) ?: return
+
+        if (expression.arguments.firstOrNull() !is FirResolvedQualifier) {
+            return
+        }
+
+        if (declarationToCheck.forwardDeclarationKindOrNull() != null) {
+            reporter.reportOn(
+                expression.source,
+                FirNativeErrors.FORWARD_DECLARATION_AS_CLASS_LITERAL,
+                expression.argument.typeRef.coneType,
+                context,
+            )
+        }
+    }
+}

compiler/fir/checkers/checkers.native/src/org/jetbrains/kotlin/fir/analysis/native/checkers/FirNativeForwardDeclarationReifiedChecker.kt

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.fir.analysis.native.checkers
+
+import org.jetbrains.kotlin.diagnostics.DiagnosticReporter
+import org.jetbrains.kotlin.diagnostics.reportOn
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.checkers.expression.FirQualifiedAccessExpressionChecker
+import org.jetbrains.kotlin.fir.analysis.diagnostics.native.FirNativeErrors
+import org.jetbrains.kotlin.fir.expressions.FirQualifiedAccessExpression
+import org.jetbrains.kotlin.fir.references.toResolvedCallableSymbol
+import org.jetbrains.kotlin.fir.types.toConeTypeProjection
+import org.jetbrains.kotlin.fir.types.toRegularClassSymbol
+import org.jetbrains.kotlin.fir.types.type
+
+object FirNativeForwardDeclarationReifiedChecker : FirQualifiedAccessExpressionChecker() {
+    override fun check(expression: FirQualifiedAccessExpression, context: CheckerContext, reporter: DiagnosticReporter) {
+        val calleeReference = expression.calleeReference
+        val typeArguments = expression.typeArguments
+        val typeParameters = calleeReference.toResolvedCallableSymbol()?.typeParameterSymbols ?: return
+
+        val count = minOf(typeArguments.size, typeParameters.size)
+        for (index in 0 until count) {
+            val typeArgumentProjection = typeArguments.elementAt(index)
+            val source = typeArgumentProjection.source ?: calleeReference.source ?: continue
+
+            val typeArgument = typeArgumentProjection.toConeTypeProjection().type ?: continue
+            val typeParameter = typeParameters[index]
+
+            if (typeParameter.isReified && typeArgument.toRegularClassSymbol(context.session)?.forwardDeclarationKindOrNull() != null) {
+                reporter.reportOn(
+                    source,
+                    FirNativeErrors.FORWARD_DECLARATION_AS_REIFIED_TYPE_ARGUMENT,
+                    typeArgument,
+                    context,
+                )
+            }
+        }
+    }
+}

compiler/fir/checkers/checkers.native/src/org/jetbrains/kotlin/fir/analysis/native/checkers/FirNativeForwardDeclarationTypeOperatorChecker.kt

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.fir.analysis.native.checkers
+
+import org.jetbrains.kotlin.diagnostics.DiagnosticReporter
+import org.jetbrains.kotlin.diagnostics.reportOn
+import org.jetbrains.kotlin.fir.analysis.checkers.*
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.checkers.expression.FirTypeOperatorCallChecker
+import org.jetbrains.kotlin.fir.analysis.diagnostics.native.FirNativeErrors
+import org.jetbrains.kotlin.fir.expressions.*
+import org.jetbrains.kotlin.fir.resolve.lookupSuperTypes
+import org.jetbrains.kotlin.fir.types.classId
+import org.jetbrains.kotlin.fir.types.coneType
+
+object FirNativeForwardDeclarationTypeOperatorChecker : FirTypeOperatorCallChecker() {
+    override fun check(expression: FirTypeOperatorCall, context: CheckerContext, reporter: DiagnosticReporter) {
+        val targetTypeRef = expression.conversionTypeRef
+        val declarationToCheck = targetTypeRef.toRegularClassSymbol(context.session) ?: return
+        val fwdKind = declarationToCheck.forwardDeclarationKindOrNull() ?: return
+
+        when (expression.operation) {
+            FirOperation.AS, FirOperation.SAFE_AS -> {
+                val sourceTypeRef = expression.argument.typeRef
+                val sourceClass = sourceTypeRef.toRegularClassSymbol(context.session)
+                // It can make sense to avoid warning if sourceClass is subclass of class with such property,
+                // but for the sake of simplicity, we don't do it now.
+                if (sourceClass != null && sourceClass.classKind == fwdKind.classKind && sourceClass.name == declarationToCheck.name) {
+                    val supers = lookupSuperTypes(
+                        symbol = sourceClass,
+                        lookupInterfaces = true,
+                        deep = true,
+                        useSiteSession = context.session,
+                    )
+                    if (supers.any { it.classId == fwdKind.matchSuperClassId }) {
+                        return
+                    }
+                }
+                reporter.reportOn(
+                    expression.source,
+                    FirNativeErrors.UNCHECKED_CAST_TO_FORWARD_DECLARATION,
+                    expression.argument.typeRef.coneType,
+                    sourceTypeRef.coneType,
+                    context,
+                )
+            }
+            FirOperation.IS, FirOperation.NOT_IS -> reporter.reportOn(
+                expression.source,
+                FirNativeErrors.CANNOT_CHECK_FOR_FORWARD_DECLARATION,
+                targetTypeRef.coneType,
+                context,
+            )
+            else -> {}
+        }
+    }
+}

compiler/fir/checkers/checkers.native/src/org/jetbrains/kotlin/fir/analysis/native/checkers/FirNativeHelpers.kt

@@ -11,7 +11,9 @@ import org.jetbrains.kotlin.diagnostics.reportOn
 import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
 import org.jetbrains.kotlin.fir.declarations.FirDeclaration
 import org.jetbrains.kotlin.fir.declarations.getAnnotationByClassId
+import org.jetbrains.kotlin.fir.symbols.impl.FirRegularClassSymbol
 import org.jetbrains.kotlin.name.ClassId
+import org.jetbrains.kotlin.name.NativeForwardDeclarationKind
 
 fun DiagnosticReporter.reportIfHasAnnotation(
     declaration: FirDeclaration,
@@ -24,3 +26,6 @@ fun DiagnosticReporter.reportIfHasAnnotation(
         reportOn(annotation.source, error, context)
     }
 }
+
+fun FirRegularClassSymbol.forwardDeclarationKindOrNull(): NativeForwardDeclarationKind? =
+    NativeForwardDeclarationKind.packageFqNameToKind[classId.packageFqName]

compiler/fir/checkers/checkers.native/src/org/jetbrains/kotlin/fir/analysis/native/checkers/NativeExpressionCheckers.kt

@@ -5,7 +5,19 @@
 
 package org.jetbrains.kotlin.fir.analysis.native.checkers
 
+import FirNativeForwardDeclarationGetClassCallChecker
 import org.jetbrains.kotlin.fir.analysis.checkers.expression.ExpressionCheckers
+import org.jetbrains.kotlin.fir.analysis.checkers.expression.FirGetClassCallChecker
+import org.jetbrains.kotlin.fir.analysis.checkers.expression.FirQualifiedAccessExpressionChecker
 
 object NativeExpressionCheckers : ExpressionCheckers() {
+    override val typeOperatorCallCheckers = setOf(
+        FirNativeForwardDeclarationTypeOperatorChecker,
+    )
+    override val getClassCallCheckers = setOf(
+        FirNativeForwardDeclarationGetClassCallChecker
+    )
+    override val qualifiedAccessExpressionCheckers = setOf(
+        FirNativeForwardDeclarationReifiedChecker
+    )
 }

compiler/testData/diagnostics/nativeTests/forwardDeclarations.kt

@@ -0,0 +1,104 @@
+// FIR_IDENTICAL
+// WITH_STDLIB
+
+// FILE: stdlib.kt
+
+package kotlinx.cinterop
+
+abstract class CStructVar
+interface ObjCObject
+abstract class ObjCObjectBase : ObjCObject
+
+// FILE: cnames.kt
+
+package cnames.structs
+
+class FwdStruct {}
+
+// FILE: objcnamesClasses.kt
+
+package objcnames.classes
+
+class FwdObjcClass {}
+
+// FILE: objcnamesProtocols.kt
+
+package objcnames.protocols
+
+interface FwdProtocol {}
+
+// FILE: lib.kt
+
+package lib
+
+import kotlinx.cinterop.*
+
+class FwdStruct : CStructVar()
+class FwdObjcClass : ObjCObjectBase()
+interface FwdProtocol : ObjCObject
+
+// FILE: lib2.kt
+
+package lib2
+
+class FwdStruct
+class FwdObjcClass
+interface FwdProtocol
+
+
+// FILE: main.kt
+
+// this inspections differs in K1/K2, but unrelated to what is tested in this test
+@file:Suppress("CAST_NEVER_SUCCEEDS", "UNUSED_PARAMETER", "UNUSED_EXPRESSION", "UNUSED_VARIABLE", "INCOMPATIBLE_TYPES")
+
+fun testUnckeckedAsFromAny(x: Any) {
+    <!UNCHECKED_CAST_TO_FORWARD_DECLARATION!>x as? cnames.structs.FwdStruct<!>
+    <!UNCHECKED_CAST_TO_FORWARD_DECLARATION!>x as? objcnames.classes.FwdObjcClass<!>
+    <!UNCHECKED_CAST_TO_FORWARD_DECLARATION!>x as? objcnames.protocols.FwdProtocol<!>
+    if (1 > 0) { <!UNCHECKED_CAST_TO_FORWARD_DECLARATION!>x as cnames.structs.FwdStruct<!> }
+    if (1 > 0) { <!UNCHECKED_CAST_TO_FORWARD_DECLARATION!>x as objcnames.classes.FwdObjcClass<!> }
+    if (1 > 0) { <!UNCHECKED_CAST_TO_FORWARD_DECLARATION!>x as objcnames.protocols.FwdProtocol<!> }
+}
+
+fun testIs(x: Any) : Int {
+    return when {
+        <!CANNOT_CHECK_FOR_FORWARD_DECLARATION!>x is cnames.structs.FwdStruct<!> -> 1
+        <!CANNOT_CHECK_FOR_FORWARD_DECLARATION!>x is objcnames.classes.FwdObjcClass<!> -> 2
+        <!CANNOT_CHECK_FOR_FORWARD_DECLARATION!>x is objcnames.protocols.FwdProtocol<!> -> 3
+        else -> 4
+    }
+}
+
+fun testIs2(x: lib.FwdStruct) = <!CANNOT_CHECK_FOR_FORWARD_DECLARATION!>x is cnames.structs.FwdStruct<!>
+fun testIs3(x: lib.FwdObjcClass) = <!CANNOT_CHECK_FOR_FORWARD_DECLARATION!>x is objcnames.classes.FwdObjcClass<!>
+fun testIs4(x: lib.FwdProtocol) = <!CANNOT_CHECK_FOR_FORWARD_DECLARATION!>x is objcnames.protocols.FwdProtocol<!>
+
+
+fun testClass1(x : cnames.structs.FwdStruct) = x::class
+fun testClass2(x : objcnames.classes.FwdObjcClass) = x::class
+fun testClass3(x : objcnames.protocols.FwdProtocol) = x::class
+fun testClass4() {
+    <!FORWARD_DECLARATION_AS_CLASS_LITERAL!>cnames.structs.FwdStruct::class<!>
+    <!FORWARD_DECLARATION_AS_CLASS_LITERAL!>objcnames.classes.FwdObjcClass::class<!>
+    <!FORWARD_DECLARATION_AS_CLASS_LITERAL!>objcnames.protocols.FwdProtocol::class<!>
+}
+inline fun <reified T> inlineF(x: T) {}
+
+fun testInline1(x : cnames.structs.FwdStruct) = <!FORWARD_DECLARATION_AS_REIFIED_TYPE_ARGUMENT!>inlineF<!>(x)
+fun testInline2(x : objcnames.classes.FwdObjcClass) = <!FORWARD_DECLARATION_AS_REIFIED_TYPE_ARGUMENT!>inlineF<!>(x)
+fun testInline3(x : objcnames.protocols.FwdProtocol) = <!FORWARD_DECLARATION_AS_REIFIED_TYPE_ARGUMENT!>inlineF<!>(x)
+fun testInline4() {
+    val a : (cnames.structs.FwdStruct) -> Unit = ::<!FORWARD_DECLARATION_AS_REIFIED_TYPE_ARGUMENT!>inlineF<!>
+    val b : (objcnames.classes.FwdObjcClass) -> Unit = ::<!FORWARD_DECLARATION_AS_REIFIED_TYPE_ARGUMENT!>inlineF<!>
+    val c : (objcnames.protocols.FwdProtocol) -> Unit = ::<!FORWARD_DECLARATION_AS_REIFIED_TYPE_ARGUMENT!>inlineF<!>
+}
+
+fun testCheckedAs1(x : lib.FwdStruct) = x as cnames.structs.FwdStruct
+fun testCheckedAs2(x : lib.FwdObjcClass) = x as objcnames.classes.FwdObjcClass
+fun testCheckedAs3(x : lib.FwdProtocol) = x as objcnames.protocols.FwdProtocol
+
+fun testUnCheckedAs1(x : lib2.FwdStruct) = <!UNCHECKED_CAST_TO_FORWARD_DECLARATION!>x as cnames.structs.FwdStruct<!>
+fun testUnCheckedAs2(x : lib2.FwdObjcClass) = <!UNCHECKED_CAST_TO_FORWARD_DECLARATION!>x as objcnames.classes.FwdObjcClass<!>
+fun testUnCheckedAs3(x : lib2.FwdProtocol) = <!UNCHECKED_CAST_TO_FORWARD_DECLARATION!>x as objcnames.protocols.FwdProtocol<!>
+
+fun testUnCheckedAs4(x : lib.FwdStruct) = <!UNCHECKED_CAST_TO_FORWARD_DECLARATION!>x as objcnames.classes.FwdObjcClass<!>

native/frontend/build.gradle.kts

@@ -6,6 +6,7 @@ plugins {
 dependencies {
     compileOnly(project(":compiler:frontend"))
     compileOnly(project(":compiler:frontend.java"))
+    compileOnly(project(":core:compiler.common.native"))
     compileOnly(intellijCore())
     api(project(":native:kotlin-native-utils"))
 }

native/frontend/src/org/jetbrains/kotlin/descriptors/konan/utils.kt

@@ -5,10 +5,15 @@
 
 package org.jetbrains.kotlin.descriptors.konan
 
+import org.jetbrains.kotlin.descriptors.ClassDescriptor
 import org.jetbrains.kotlin.descriptors.ModuleDescriptor
+import org.jetbrains.kotlin.descriptors.PackageFragmentDescriptor
 import org.jetbrains.kotlin.konan.library.KONAN_STDLIB_NAME
+import org.jetbrains.kotlin.name.NativeForwardDeclarationKind
 import org.jetbrains.kotlin.name.Name
 
 val NATIVE_STDLIB_MODULE_NAME = Name.special("<$KONAN_STDLIB_NAME>")
 
 fun ModuleDescriptor.isNativeStdlib() = name == NATIVE_STDLIB_MODULE_NAME
+fun ClassDescriptor.getForwardDeclarationKindOrNull() =
+    NativeForwardDeclarationKind.packageFqNameToKind[(containingDeclaration as? PackageFragmentDescriptor)?.fqName]

native/frontend/src/org/jetbrains/kotlin/resolve/konan/diagnostics/DefaultErrorMessagesNative.kt

@@ -69,6 +69,27 @@ private val DIAGNOSTIC_FACTORY_TO_RENDERER by lazy {
             ErrorsNative.SUBTYPE_OF_HIDDEN_FROM_OBJC,
             "Only @HiddenFromObjC declaration can be a subtype of @HiddenFromObjC declaration"
         )
+
+        put(
+            ErrorsNative.CANNOT_CHECK_FOR_FORWARD_DECLARATION,
+            "Cannot check for forward declaration: ''{0}''",
+            Renderers.RENDER_TYPE
+        )
+        put(ErrorsNative.UNCHECKED_CAST_TO_FORWARD_DECLARATION,
+            "Unchecked cast to forward declaration: ''{0}'' to ''{1}''",
+            Renderers.RENDER_TYPE,
+            Renderers.RENDER_TYPE
+        )
+        put(
+            ErrorsNative.FORWARD_DECLARATION_AS_REIFIED_TYPE_ARGUMENT,
+            "Cannot pass forward declaration ''{0}'' for reified type parameter",
+            Renderers.RENDER_TYPE
+        )
+        put(
+            ErrorsNative.FORWARD_DECLARATION_AS_CLASS_LITERAL,
+            "Can't refer to forward declaration ''{0}'' from class literal",
+            Renderers.RENDER_TYPE
+        )
     }
 }
 

native/frontend/src/org/jetbrains/kotlin/resolve/konan/diagnostics/ErrorsNative.kt

@@ -12,6 +12,7 @@ import org.jetbrains.kotlin.diagnostics.*
 import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.psi.KtDeclaration
 import org.jetbrains.kotlin.psi.KtElement
+import org.jetbrains.kotlin.types.KotlinType
 
 object ErrorsNative {
     @JvmField
@@ -60,6 +61,14 @@ object ErrorsNative {
     val INVALID_REFINES_IN_SWIFT_TARGETS = DiagnosticFactory0.create<KtElement>(Severity.ERROR)
     @JvmField
     val SUBTYPE_OF_HIDDEN_FROM_OBJC = DiagnosticFactory0.create<KtElement>(Severity.ERROR)
+    @JvmField
+    val CANNOT_CHECK_FOR_FORWARD_DECLARATION = DiagnosticFactory1.create<PsiElement, KotlinType>(Severity.ERROR)
+    @JvmField
+    val UNCHECKED_CAST_TO_FORWARD_DECLARATION = DiagnosticFactory2.create<PsiElement, KotlinType, KotlinType>(Severity.WARNING)
+    @JvmField
+    val FORWARD_DECLARATION_AS_REIFIED_TYPE_ARGUMENT = DiagnosticFactory1.create<PsiElement, KotlinType>(Severity.ERROR)
+    @JvmField
+    val FORWARD_DECLARATION_AS_CLASS_LITERAL = DiagnosticFactory1.create<PsiElement, KotlinType>(Severity.ERROR)
 
     init {
         Errors.Initializer.initializeFactoryNames(ErrorsNative::class.java)

native/frontend/src/org/jetbrains/kotlin/resolve/konan/diagnostics/NativeForwardDeclarationRttiChecker.kt

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.resolve.konan.diagnostics
+
+import com.intellij.psi.PsiElement
+import org.jetbrains.kotlin.descriptors.ClassDescriptor
+import org.jetbrains.kotlin.descriptors.konan.getForwardDeclarationKindOrNull
+import org.jetbrains.kotlin.psi.KtClassLiteralExpression
+import org.jetbrains.kotlin.resolve.BindingTrace
+import org.jetbrains.kotlin.resolve.calls.checkers.RttiExpressionChecker
+import org.jetbrains.kotlin.resolve.calls.checkers.RttiExpressionInformation
+import org.jetbrains.kotlin.resolve.calls.checkers.RttiOperation
+import org.jetbrains.kotlin.resolve.calls.context.ResolutionContext
+import org.jetbrains.kotlin.resolve.descriptorUtil.fqNameSafe
+import org.jetbrains.kotlin.resolve.descriptorUtil.getAllSuperClassifiers
+import org.jetbrains.kotlin.types.KotlinType
+import org.jetbrains.kotlin.types.expressions.ClassLiteralChecker
+
+class NativeForwardDeclarationRttiChecker : RttiExpressionChecker, ClassLiteralChecker {
+    override fun check(rttiInformation: RttiExpressionInformation, reportOn: PsiElement, trace: BindingTrace) {
+        val sourceType = rttiInformation.sourceType
+        val targetType = rttiInformation.targetType
+        val targetDescriptor = targetType?.constructor?.declarationDescriptor
+        if (sourceType != null && targetDescriptor is ClassDescriptor) {
+            val kind = targetDescriptor.getForwardDeclarationKindOrNull() ?: return
+            when (rttiInformation.operation) {
+                RttiOperation.IS,
+                RttiOperation.NOT_IS -> trace.report(ErrorsNative.CANNOT_CHECK_FOR_FORWARD_DECLARATION.on(reportOn, targetType))
+
+                RttiOperation.AS,
+                RttiOperation.SAFE_AS -> {
+                    // It can make sense to avoid warning if sourceClass is subclass of class with such property,
+                    // but for the sake of simplicity, we don't do it now.
+                    val sourceDescriptor = sourceType.constructor.declarationDescriptor as? ClassDescriptor
+                    val isAllowedCast = sourceDescriptor != null &&
+                            sourceDescriptor.name == targetDescriptor.name &&
+                            sourceDescriptor.kind == kind.classKind &&
+                            sourceDescriptor.getAllSuperClassifiers().any { it.fqNameSafe == kind.matchSuperClassFqName }
+                    if (!isAllowedCast) {
+                        trace.report(ErrorsNative.UNCHECKED_CAST_TO_FORWARD_DECLARATION.on(reportOn, sourceType, targetType))
+                    }
+                }
+            }
+        }
+    }
+
+    override fun check(expression: KtClassLiteralExpression, type: KotlinType, context: ResolutionContext<*>) {
+        val descriptor = type.constructor.declarationDescriptor as? ClassDescriptor
+        if (descriptor?.getForwardDeclarationKindOrNull() != null) {
+            context.trace.report(ErrorsNative.FORWARD_DECLARATION_AS_CLASS_LITERAL.on(expression, type))
+        }
+    }
+}

native/frontend/src/org/jetbrains/kotlin/resolve/konan/diagnostics/NativeReifiedForwardDeclarationChecker.kt

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.resolve.konan.diagnostics
+
+import com.intellij.psi.PsiElement
+import org.jetbrains.kotlin.descriptors.ClassDescriptor
+import org.jetbrains.kotlin.resolve.calls.checkers.CallChecker
+import org.jetbrains.kotlin.resolve.calls.checkers.CallCheckerContext
+import org.jetbrains.kotlin.resolve.calls.model.ResolvedCall
+import org.jetbrains.kotlin.descriptors.konan.*
+
+class NativeReifiedForwardDeclarationChecker : CallChecker {
+    override fun check(resolvedCall: ResolvedCall<*>, reportOn: PsiElement, context: CallCheckerContext) {
+        val typeArgumentList = resolvedCall.call.typeArgumentList?.arguments
+        for ((typeParam, typeArg) in resolvedCall.typeArguments) {
+            if (!typeParam.isReified) continue
+
+            val typeArgDescriptor = typeArg.constructor.declarationDescriptor
+            if (typeArgDescriptor is ClassDescriptor && typeArgDescriptor.getForwardDeclarationKindOrNull() != null) {
+                val typeArgumentPsi = if (typeArgumentList != null) {
+                    typeArgumentList[typeParam.index].typeReference
+                } else {
+                    resolvedCall.call.calleeExpression
+                }
+
+                context.trace.report(ErrorsNative.FORWARD_DECLARATION_AS_REIFIED_TYPE_ARGUMENT.on(typeArgumentPsi!!, typeArg))
+            }
+        }
+    }
+}

native/frontend/src/org/jetbrains/kotlin/resolve/konan/platform/NativePlatformConfigurator.kt

@@ -21,19 +21,21 @@ import org.jetbrains.kotlin.resolve.konan.diagnostics.*
 object NativePlatformConfigurator : PlatformConfiguratorBase(
     additionalCallCheckers = listOf(
         SuperCallWithDefaultArgumentsChecker(),
-        LateinitIntrinsicApplicabilityChecker(isWarningInPre19 = true)
+        LateinitIntrinsicApplicabilityChecker(isWarningInPre19 = true),
+        NativeReifiedForwardDeclarationChecker(),
     ),
     additionalDeclarationCheckers = listOf(
         NativeThrowsChecker, NativeSharedImmutableChecker,
         NativeThreadLocalChecker,
         NativeObjCNameChecker, NativeObjCNameOverridesChecker,
         NativeObjCRefinementChecker, NativeObjCRefinementAnnotationChecker,
-        NativeObjCRefinementOverridesChecker, NativeHiddenFromObjCInheritanceChecker
+        NativeObjCRefinementOverridesChecker, NativeHiddenFromObjCInheritanceChecker,
     )
 ) {
     override fun configureModuleComponents(container: StorageComponentContainer) {
         container.useInstance(NativeInliningRule)
         container.useImpl<NativeIdentifierChecker>()
+        container.useImpl<NativeForwardDeclarationRttiChecker>()
     }
 
     override fun configureModuleDependentCheckers(container: StorageComponentContainer) {

native/native.tests/tests-gen/org/jetbrains/kotlin/konan/diagnostics/DiagnosticsNativeTestGenerated.java

@@ -30,6 +30,12 @@ public class DiagnosticsNativeTestGenerated extends AbstractDiagnosticsNativeTes
         runTest("compiler/testData/diagnostics/nativeTests/cloneableInNative.kt");
     }
 
+    @Test
+    @TestMetadata("forwardDeclarations.kt")
+    public void testForwardDeclarations() throws Exception {
+        runTest("compiler/testData/diagnostics/nativeTests/forwardDeclarations.kt");
+    }
+
     @Test
     @TestMetadata("identifiers.kt")
     public void testIdentifiers() throws Exception {

native/native.tests/tests-gen/org/jetbrains/kotlin/konan/diagnostics/FirLightTreeOldFrontendNativeDiagnosticsTestGenerated.java

@@ -34,6 +34,12 @@ public class FirLightTreeOldFrontendNativeDiagnosticsTestGenerated extends Abstr
         runTest("compiler/testData/diagnostics/nativeTests/cloneableInNative.kt");
     }
 
+    @Test
+    @TestMetadata("forwardDeclarations.kt")
+    public void testForwardDeclarations() throws Exception {
+        runTest("compiler/testData/diagnostics/nativeTests/forwardDeclarations.kt");
+    }
+
     @Test
     @TestMetadata("identifiers.kt")
     public void testIdentifiers() throws Exception {

native/native.tests/tests-gen/org/jetbrains/kotlin/konan/diagnostics/FirPsiOldFrontendNativeDiagnosticsTestGenerated.java

@@ -34,6 +34,12 @@ public class FirPsiOldFrontendNativeDiagnosticsTestGenerated extends AbstractFir
         runTest("compiler/testData/diagnostics/nativeTests/cloneableInNative.kt");
     }
 
+    @Test
+    @TestMetadata("forwardDeclarations.kt")
+    public void testForwardDeclarations() throws Exception {
+        runTest("compiler/testData/diagnostics/nativeTests/forwardDeclarations.kt");
+    }
+
     @Test
     @TestMetadata("identifiers.kt")
     public void testIdentifiers() throws Exception {