JVM, JVM IR: report error if not all parts of multifile class are @JvmSynthetic

#KT-41884 Fixed

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/GenerateMultifileFacades.kt

@@ -17,6 +17,7 @@ import org.jetbrains.kotlin.backend.common.phaser.makeCustomPhase
 import org.jetbrains.kotlin.backend.jvm.JvmBackendContext
 import org.jetbrains.kotlin.backend.jvm.JvmLoweredDeclarationOrigin
 import org.jetbrains.kotlin.backend.jvm.codegen.fileParent
+import org.jetbrains.kotlin.backend.jvm.ir.getKtFile
 import org.jetbrains.kotlin.config.JvmAnalysisFlags
 import org.jetbrains.kotlin.descriptors.DescriptorVisibilities
 import org.jetbrains.kotlin.descriptors.Modality
@@ -42,6 +43,8 @@ import org.jetbrains.kotlin.ir.visitors.transformChildrenVoid
 import org.jetbrains.kotlin.load.java.JavaDescriptorVisibilities
 import org.jetbrains.kotlin.resolve.inline.INLINE_ONLY_ANNOTATION_FQ_NAME
 import org.jetbrains.kotlin.resolve.jvm.JvmClassName
+import org.jetbrains.kotlin.resolve.jvm.annotations.JVM_SYNTHETIC_ANNOTATION_FQ_NAME
+import org.jetbrains.kotlin.resolve.jvm.diagnostics.ErrorsJvm
 
 internal val generateMultifileFacadesPhase = makeCustomPhase<JvmBackendContext, IrModuleFragment>(
     name = "GenerateMultifileFacades",
@@ -133,7 +136,22 @@ private fun generateMultifileFacades(
                     }
                 }
             }
+
+            val nonJvmSyntheticParts = partClasses.filterNot { it.hasAnnotation(JVM_SYNTHETIC_ANNOTATION_FQ_NAME) }
+            if (nonJvmSyntheticParts.isEmpty()) {
+                annotations = annotations + partClasses.first().getAnnotation(JVM_SYNTHETIC_ANNOTATION_FQ_NAME)!!.deepCopyWithSymbols()
+            } else if (nonJvmSyntheticParts.size < partClasses.size) {
+                for (part in nonJvmSyntheticParts) {
+                    val partFile = part.fileParent.getKtFile() ?: error("Not a KtFile: ${part.render()} ${part.fileParent}")
+                    // If at least one of parts is annotated with @JvmSynthetic, then all other parts should also be annotated.
+                    // We report this error on the package directive for each non-@JvmSynthetic part.
+                    context.state.diagnostics.report(
+                        ErrorsJvm.NOT_ALL_MULTIFILE_CLASS_PARTS_ARE_JVM_SYNTHETIC.on(partFile.packageDirective ?: partFile)
+                    )
+                }
+            }
         }
+
         file.declarations.add(facadeClass)
 
         for (partClass in partClasses) {