K2: report JvmMultifileClass+JvmSynthetic error in JVM backend

Specifically, the case when not all parts of a multifile class are
annotated with `@JvmSynthetic`. Report the error on the
`@JvmMultifileClass` annotation instead of the package directive,
because the latter is difficult to find via IR. This fixes the test
FirLightTreeDiagnosticsTestWithJvmIrBackendGenerated.MultifileClasses.testJvmSynthetic.

 #KT-59586

compiler/frontend.java/src/org/jetbrains/kotlin/resolve/jvm/diagnostics/JvmBackendErrors.kt

@@ -37,6 +37,8 @@ object JvmBackendErrors {
 
     val INLINE_CALL_CYCLE by error1<PsiElement, Name>()
 
+    val NOT_ALL_MULTIFILE_CLASS_PARTS_ARE_JVM_SYNTHETIC by error0<PsiElement>()
+
     init {
         RootDiagnosticRendererFactory.registerFactory(KtDefaultJvmErrorMessages)
     }
@@ -72,5 +74,9 @@ object KtDefaultJvmErrorMessages : BaseDiagnosticRendererFactory() {
         map.put(JvmBackendErrors.SCRIPT_CAPTURING_ENUM_ENTRY, "Enum entry {0} captures the script class instance. Try to use class or anonymous object instead", STRING)
 
         map.put(JvmBackendErrors.INLINE_CALL_CYCLE, "The ''{0}'' invocation is a part of inline cycle", NAME)
+        map.put(
+            JvmBackendErrors.NOT_ALL_MULTIFILE_CLASS_PARTS_ARE_JVM_SYNTHETIC,
+            "All of multi-file class parts should be annotated with @JvmSynthetic if at least one of them is"
+        )
     }
 }

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

@@ -14,7 +14,6 @@ import org.jetbrains.kotlin.backend.jvm.JvmBackendContext
 import org.jetbrains.kotlin.backend.jvm.JvmLoweredDeclarationOrigin
 import org.jetbrains.kotlin.backend.jvm.MultifileFacadeFileEntry
 import org.jetbrains.kotlin.backend.jvm.ir.fileParent
-import org.jetbrains.kotlin.backend.jvm.ir.getKtFile
 import org.jetbrains.kotlin.backend.jvm.isMultifileBridge
 import org.jetbrains.kotlin.config.JvmAnalysisFlags
 import org.jetbrains.kotlin.descriptors.DescriptorVisibilities
@@ -38,9 +37,10 @@ import org.jetbrains.kotlin.ir.visitors.IrElementTransformerVoid
 import org.jetbrains.kotlin.ir.visitors.IrElementVisitor
 import org.jetbrains.kotlin.ir.visitors.transformChildrenVoid
 import org.jetbrains.kotlin.load.java.JavaDescriptorVisibilities
+import org.jetbrains.kotlin.name.JvmNames
 import org.jetbrains.kotlin.name.JvmNames.JVM_SYNTHETIC_ANNOTATION_FQ_NAME
 import org.jetbrains.kotlin.resolve.inline.INLINE_ONLY_ANNOTATION_FQ_NAME
-import org.jetbrains.kotlin.resolve.jvm.diagnostics.ErrorsJvm
+import org.jetbrains.kotlin.resolve.jvm.diagnostics.JvmBackendErrors
 
 internal val generateMultifileFacadesPhase = makeCustomPhase<JvmBackendContext, IrModuleFragment>(
     name = "GenerateMultifileFacades",
@@ -118,11 +118,12 @@ private fun generateMultifileFacades(
                 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}")
+                    val partFile = 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)
+                    // We report this error on the `@JvmMultifileClass` annotation of each non-@JvmSynthetic part.
+                    val annotation = partFile.annotations.singleOrNull { it.isAnnotationWithEqualFqName(JvmNames.JVM_MULTIFILE_CLASS) }
+                    context.ktDiagnosticReporter.at(annotation ?: partFile, partFile).report(
+                        JvmBackendErrors.NOT_ALL_MULTIFILE_CLASS_PARTS_ARE_JVM_SYNTHETIC
                     )
                 }
             }

compiler/testData/diagnostics/testsWithJvmBackend/multifileClasses/jvmSynthetic.kt

@@ -3,8 +3,8 @@
 // FILE: f.kt
 
 @file:JvmName("Foo")
-@file:JvmMultifileClass
-<!NOT_ALL_MULTIFILE_CLASS_PARTS_ARE_JVM_SYNTHETIC!>package test<!>
+<!NOT_ALL_MULTIFILE_CLASS_PARTS_ARE_JVM_SYNTHETIC!>@file:JvmMultifileClass<!>
+package test
 
 fun f() {}
 
@@ -20,8 +20,8 @@ val g = ""
 // FILE: h.kt
 
 @file:JvmName("Foo")
-@file:JvmMultifileClass
-<!NOT_ALL_MULTIFILE_CLASS_PARTS_ARE_JVM_SYNTHETIC!>package test<!>
+<!NOT_ALL_MULTIFILE_CLASS_PARTS_ARE_JVM_SYNTHETIC!>@file:JvmMultifileClass<!>
+package test
 
 fun h() {}