JS: prohibit non-abstract members of external interfaces, except for nullable properties. See KT-15308

js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/DefaultErrorMessagesJs.kt

@@ -73,6 +73,8 @@ private val DIAGNOSTIC_FACTORY_TO_RENDERER by lazy {
             "Overriding `external` function with optional parameters by declaration from superclass: {0}", Renderers.COMPACT)
         put(ErrorsJs.IMPLEMENTING_FUNCTION_INTERFACE, "Implementing function interface is prohibited in JavaScript")
         put(ErrorsJs.INLINE_EXTERNAL_DECLARATION, "Inline external declaration")
+        put(ErrorsJs.NON_ABSTRACT_MEMBER_OF_EXTERNAL_INTERFACE,
+            "Only nullable properties of external interfaces are allowed to be non-abstract")
 
         this
     }

js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/ErrorsJs.java

@@ -78,6 +78,8 @@ public interface ErrorsJs {
             ERROR, PositioningStrategies.DECLARATION_SIGNATURE_OR_DEFAULT);
     DiagnosticFactory0<KtDeclaration> INLINE_EXTERNAL_DECLARATION = DiagnosticFactory0.create(
             ERROR, PositioningStrategies.DECLARATION_SIGNATURE_OR_DEFAULT);
+    DiagnosticFactory0<KtDeclaration> NON_ABSTRACT_MEMBER_OF_EXTERNAL_INTERFACE = DiagnosticFactory0.create(
+            ERROR, PositioningStrategies.DECLARATION_SIGNATURE_OR_DEFAULT);
 
     @SuppressWarnings("UnusedDeclaration")
     Object _initializer = new Object() {

js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsExternalChecker.kt

@@ -18,7 +18,6 @@ package org.jetbrains.kotlin.js.resolve.diagnostics
 
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns
 import org.jetbrains.kotlin.descriptors.*
-import org.jetbrains.kotlin.descriptors.annotations.isInlineOnlyOrReified
 import org.jetbrains.kotlin.diagnostics.DiagnosticSink
 import org.jetbrains.kotlin.js.PredefinedAnnotation
 import org.jetbrains.kotlin.js.translate.utils.AnnotationsUtils
@@ -80,6 +79,12 @@ object JsExternalChecker : SimpleDeclarationChecker {
         if (descriptor is FunctionDescriptor && descriptor.isInline) {
             diagnosticHolder.report(ErrorsJs.INLINE_EXTERNAL_DECLARATION.on(declaration))
         }
+
+        if (descriptor is CallableMemberDescriptor && descriptor.isNonAbstractMemberOfInterface() &&
+            !descriptor.isNullableProperty()
+        ) {
+            diagnosticHolder.report(ErrorsJs.NON_ABSTRACT_MEMBER_OF_EXTERNAL_INTERFACE.on(declaration))
+        }
     }
 
     private fun isDirectlyExternal(declaration: KtDeclaration, descriptor: DeclarationDescriptor): Boolean {
@@ -96,4 +101,31 @@ object JsExternalChecker : SimpleDeclarationChecker {
         val containingDeclaration = descriptor.containingDeclaration as? ClassDescriptor ?: return false
         return AnnotationsUtils.isNativeObject(containingDeclaration)
     }
+
+    private fun CallableMemberDescriptor.isNonAbstractMemberOfInterface() =
+            modality != Modality.ABSTRACT && DescriptorUtils.isInterface(containingDeclaration) &&
+            this !is PropertyAccessorDescriptor
+
+    private fun CallableMemberDescriptor.isNullableProperty() = this is PropertyDescriptor && TypeUtils.isNullableType(type)
+
+    private fun KtDeclarationWithBody.hasValidExternalBody(bindingContext: BindingContext): Boolean {
+        if (!hasBody()) return true
+        val body = bodyExpression!!
+        return if (!hasBlockBody()) {
+            body.isNoImplExpression(bindingContext)
+        }
+        else if (body is KtBlockExpression) {
+            val statement = body.statements.singleOrNull() ?: return false
+            statement.isNoImplExpression(bindingContext)
+        }
+        else {
+            false
+        }
+    }
+
+    private fun KtExpression.isNoImplExpression(bindingContext: BindingContext): Boolean {
+        val descriptor = getResolvedCall(bindingContext)?.resultingDescriptor as? PropertyDescriptor ?: return false
+        val container = descriptor.containingDeclaration as? PackageFragmentDescriptor ?: return false
+        return container.fqNameUnsafe == NO_IMPL_PROPERTY_NAME.parent() && descriptor.name == NO_IMPL_PROPERTY_NAME.shortName()
+    }
 }