diff --git a/compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/codegen/ClassCodegen.kt b/compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/codegen/ClassCodegen.kt
index 6d2a8f09959..c4d8e6dade6 100644
--- a/compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/codegen/ClassCodegen.kt
+++ b/compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/codegen/ClassCodegen.kt
@@ -35,13 +35,13 @@ import org.jetbrains.kotlin.load.java.JvmAbi
 import org.jetbrains.kotlin.load.java.JvmAnnotationNames
 import org.jetbrains.kotlin.load.kotlin.header.KotlinClassHeader
 import org.jetbrains.kotlin.name.Name
-import org.jetbrains.kotlin.name.SpecialNames
 import org.jetbrains.kotlin.resolve.jvm.AsmTypes
 import org.jetbrains.kotlin.resolve.jvm.annotations.JVM_SYNTHETIC_ANNOTATION_FQ_NAME
 import org.jetbrains.kotlin.resolve.jvm.annotations.TRANSIENT_ANNOTATION_FQ_NAME
 import org.jetbrains.kotlin.resolve.jvm.annotations.VOLATILE_ANNOTATION_FQ_NAME
 import org.jetbrains.kotlin.resolve.jvm.diagnostics.JvmDeclarationOrigin
 import org.jetbrains.kotlin.resolve.jvm.diagnostics.OtherOrigin
+import org.jetbrains.kotlin.resolve.jvm.jvmSignature.JvmClassSignature
 import org.jetbrains.kotlin.serialization.DescriptorSerializer
 import org.jetbrains.kotlin.utils.addToStdlib.safeAs
 import org.jetbrains.org.objectweb.asm.Opcodes
@@ -110,6 +110,13 @@ open class ClassCodegen protected constructor(
         val superClassInfo = irClass.getSuperClassInfo(typeMapper)
         val signature = getSignature(irClass, type, superClassInfo, typeMapper)
 
+        // Ensure that the backend only produces class names that would be valid in the frontend for JVM.
+        // hasInvalidName() uses a set copied from JvmSimpleNameBacktickChecker, which in turn references JVMS 4.7.9.1:
+        // https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.7.9.1.
+        if (context.state.classBuilderMode.generateBodies && signature.hasInvalidName()) {
+            throw IllegalStateException("Generating class with invalid name '${type.className}': ${irClass.dump()}")
+        }
+
         visitor.defineClass(
             irClass.descriptor.psiElement,
             state.classFileVersion,
@@ -281,20 +288,14 @@ open class ClassCodegen protected constructor(
 
     companion object {
         fun generate(irClass: IrClass, context: JvmBackendContext) {
-            val state = context.state
-
-            if (irClass.name == SpecialNames.NO_NAME_PROVIDED) {
-                badClass(irClass, state.classBuilderMode)
-            }
-
             ClassCodegen(irClass, context).generate()
         }
 
-        private fun badClass(irClass: IrClass, mode: ClassBuilderMode) {
-            if (mode.generateBodies) {
-                throw IllegalStateException("Generating bad class in ClassBuilderMode = $mode: ${irClass.dump()}")
-            }
-        }
+        // Note: This is copied from JvmSimpleNameBacktickChecker.
+        private val INVALID_CHARS = setOf('.', ';', '[', ']', '/', '<', '>', ':', '\\')
+
+        private fun JvmClassSignature.hasInvalidName() =
+            name.splitToSequence('/').any { identifier -> identifier.any { it in INVALID_CHARS } }
     }
 
     private fun generateDeclaration(declaration: IrDeclaration) {
diff --git a/compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/InventNamesForLocalClasses.kt b/compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/InventNamesForLocalClasses.kt
index 92126bb5437..eccf77f616c 100644
--- a/compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/InventNamesForLocalClasses.kt
+++ b/compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/InventNamesForLocalClasses.kt
@@ -17,6 +17,7 @@ import org.jetbrains.kotlin.ir.symbols.IrFunctionSymbol
 import org.jetbrains.kotlin.ir.util.isAnonymousObject
 import org.jetbrains.kotlin.ir.visitors.IrElementVisitor
 import org.jetbrains.kotlin.name.Name
+import org.jetbrains.kotlin.name.NameUtils
 import org.jetbrains.kotlin.resolve.jvm.JvmClassName
 import org.jetbrains.org.objectweb.asm.Type
 
@@ -102,10 +103,11 @@ class InventNamesForLocalClasses(private val context: JvmBackendContext) : FileL
             val simpleName = declaration.name.asString()
 
             val internalName = when {
-                declaration.origin == IrDeclarationOrigin.LOCAL_FUNCTION_FOR_LAMBDA -> {
+                declaration is IrFunction && !NameUtils.hasName(declaration.name) -> {
+                    // Replace "unnamed" function names with indices.
                     inventName(null, data).also { name ->
-                        // We save the name of the lambda to reuse it in the reference to it (produced by the closure conversion) later.
-                        localFunctionNames[(declaration as IrFunction).symbol] = name
+                        // We save the name of the function to reuse it in the reference to it (produced by the closure conversion) later.
+                        localFunctionNames[declaration.symbol] = name
                     }
                 }
                 enclosingName != null -> "$enclosingName$$simpleName"