Inline classes can only implement interfaces

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

@@ -328,6 +328,7 @@ public interface Errors {
     DiagnosticFactory0<PsiElement> DELEGATED_PROPERTY_INSIDE_INLINE_CLASS = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory1<KtTypeReference, KotlinType> INLINE_CLASS_HAS_INAPPLICABLE_PARAMETER_TYPE = DiagnosticFactory1.create(ERROR);
     DiagnosticFactory0<PsiElement> INLINE_CLASS_CANNOT_IMPLEMENT_INTERFACE_BY_DELEGATION = DiagnosticFactory0.create(ERROR);
+    DiagnosticFactory0<KtTypeReference> INLINE_CLASS_CANNOT_EXTEND_CLASSES = DiagnosticFactory0.create(ERROR);
 
     // Secondary constructors
 

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java

@@ -657,6 +657,7 @@ public class DefaultErrorMessages {
         MAP.put(DELEGATED_PROPERTY_INSIDE_INLINE_CLASS, "Inline class cannot have delegated properties");
         MAP.put(INLINE_CLASS_HAS_INAPPLICABLE_PARAMETER_TYPE, "Inline class cannot have value parameter of type ''{0}''", RENDER_TYPE);
         MAP.put(INLINE_CLASS_CANNOT_IMPLEMENT_INTERFACE_BY_DELEGATION, "Inline class cannot implement an interface by delegation");
+        MAP.put(INLINE_CLASS_CANNOT_EXTEND_CLASSES, "Inline class cannot extend classes");
 
         MAP.put(VARIANCE_ON_TYPE_PARAMETER_NOT_ALLOWED, "Variance annotations are only allowed for type parameters of classes and interfaces");
         MAP.put(BOUND_ON_TYPE_ALIAS_PARAMETER_NOT_ALLOWED, "Bounds are not allowed on type alias parameters");

compiler/frontend/src/org/jetbrains/kotlin/resolve/checkers/InlineClassDeclarationChecker.kt

@@ -92,6 +92,14 @@ object InlineClassDeclarationChecker : DeclarationChecker {
             if (supertypeEntry is KtDelegatedSuperTypeEntry) {
                 trace.report(Errors.INLINE_CLASS_CANNOT_IMPLEMENT_INTERFACE_BY_DELEGATION.on(supertypeEntry))
                 return
+            } else {
+                val typeReference = supertypeEntry.typeReference ?: continue
+                val type = trace[BindingContext.TYPE, typeReference] ?: continue
+                val typeDescriptor = type.constructor.declarationDescriptor ?: continue
+                if (!DescriptorUtils.isInterface(typeDescriptor)) {
+                    trace.report(Errors.INLINE_CLASS_CANNOT_EXTEND_CLASSES.on(typeReference))
+                    return
+                }
             }
         }
     }

compiler/testData/diagnostics/tests/inlineClasses/inlineClassCanOnlyImplementInterfaces.kt

@@ -0,0 +1,13 @@
+// !LANGUAGE: +InlineClasses
+
+abstract class AbstractBaseClass
+
+open class OpenBaseClass
+
+interface BaseInterface
+
+inline class TestExtendsAbstractClass(val x: Int) : <!INLINE_CLASS_CANNOT_EXTEND_CLASSES!>AbstractBaseClass<!>()
+
+inline class TestExtendsOpenClass(val x: Int) : <!INLINE_CLASS_CANNOT_EXTEND_CLASSES!>OpenBaseClass<!>()
+
+inline class TestImplementsInterface(val x: Int) : BaseInterface

compiler/testData/diagnostics/tests/inlineClasses/inlineClassCanOnlyImplementInterfaces.txt

@@ -0,0 +1,45 @@
+package
+
+public abstract class AbstractBaseClass {
+    public constructor AbstractBaseClass()
+    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
+}
+
+public interface BaseInterface {
+    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
+}
+
+public open class OpenBaseClass {
+    public constructor OpenBaseClass()
+    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
+}
+
+public final inline class TestExtendsAbstractClass : AbstractBaseClass {
+    public constructor TestExtendsAbstractClass(/*0*/ x: kotlin.Int)
+    public final val x: kotlin.Int
+    public open override /*1*/ /*synthesized*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*synthesized*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*synthesized*/ fun toString(): kotlin.String
+}
+
+public final inline class TestExtendsOpenClass : OpenBaseClass {
+    public constructor TestExtendsOpenClass(/*0*/ x: kotlin.Int)
+    public final val x: kotlin.Int
+    public open override /*1*/ /*synthesized*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*synthesized*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*synthesized*/ fun toString(): kotlin.String
+}
+
+public final inline class TestImplementsInterface : BaseInterface {
+    public constructor TestImplementsInterface(/*0*/ x: kotlin.Int)
+    public final val x: kotlin.Int
+    public open override /*1*/ /*synthesized*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*synthesized*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*synthesized*/ fun toString(): kotlin.String
+}

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

@@ -10867,6 +10867,11 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                 runTest("compiler/testData/diagnostics/tests/inlineClasses/identityComparisonWithInlineClasses.kt");
             }
 
+            @TestMetadata("inlineClassCanOnlyImplementInterfaces.kt")
+            public void testInlineClassCanOnlyImplementInterfaces() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/inlineClasses/inlineClassCanOnlyImplementInterfaces.kt");
+            }
+
             @TestMetadata("inlineClassCannotImplementInterfaceByDelegation.kt")
             public void testInlineClassCannotImplementInterfaceByDelegation() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/inlineClasses/inlineClassCannotImplementInterfaceByDelegation.kt");

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

@@ -10867,6 +10867,11 @@ public class DiagnosticsUsingJavacTestGenerated extends AbstractDiagnosticsUsing
                 runTest("compiler/testData/diagnostics/tests/inlineClasses/identityComparisonWithInlineClasses.kt");
             }
 
+            @TestMetadata("inlineClassCanOnlyImplementInterfaces.kt")
+            public void testInlineClassCanOnlyImplementInterfaces() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/inlineClasses/inlineClassCanOnlyImplementInterfaces.kt");
+            }
+
             @TestMetadata("inlineClassCannotImplementInterfaceByDelegation.kt")
             public void testInlineClassCannotImplementInterfaceByDelegation() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/inlineClasses/inlineClassCannotImplementInterfaceByDelegation.kt");