Kotlin Help

What's new in Kotlin 1.6.0

Released: 16 November 2021

Kotlin 1.6.0 introduces new language features, optimizations and improvements to existing features, and a lot of improvements to the Kotlin standard library.

You can also find an overview of the changes in the release blog post.


Kotlin 1.6.0 brings stabilization to several language features introduced for preview in the previous 1.5.30 release:

It also includes various type inference improvements and support for annotations on class type parameters:

Stable exhaustive when statements for enum, sealed, and Boolean subjects

An exhaustive when statement contains branches for all possible types or values of its subject, or for some types plus an else branch. It covers all possible cases, making your code safer.

We will soon prohibit non-exhaustive when statements to make the behavior consistent with when expressions. To ensure smooth migration, Kotlin 1.6.0 reports warnings about non-exhaustive when statements with an enum, sealed, or Boolean subject. These warnings will become errors in future releases.

sealed class Contact { data class PhoneCall(val number: String) : Contact() data class TextMessage(val number: String) : Contact() } fun Contact.messageCost(): Int = when(this) { // Error: 'when' expression must be exhaustive is Contact.PhoneCall -> 42 } fun sendMessage(contact: Contact, message: String) { // Starting with 1.6.0 // Warning: Non exhaustive 'when' statements on Boolean will be // prohibited in 1.7, add 'false' branch or 'else' branch instead when(message.isEmpty()) { true -> return } // Warning: Non exhaustive 'when' statements on sealed class/interface will be // prohibited in 1.7, add 'is TextMessage' branch or 'else' branch instead when(contact) { is Contact.PhoneCall -> TODO() } }

See this YouTrack ticket for a more detailed explanation of the change and its effects.

Stable suspending functions as supertypes

Implementation of suspending functional types has become Stable in Kotlin 1.6.0. A preview was available in 1.5.30.

The feature can be useful when designing APIs that use Kotlin coroutines and accept suspending functional types. You can now streamline your code by enclosing the desired behavior in a separate class that implements a suspending functional type.

class MyClickAction : suspend () -> Unit { override suspend fun invoke() { TODO() } } fun launchOnClick(action: suspend () -> Unit) {}

You can use an instance of this class where only lambdas and suspending function references were allowed previously: launchOnClick(MyClickAction()).

There are currently two limitations coming from implementation details:

  • You can't mix ordinary functional types and suspending ones in the list of supertypes.

  • You can't use multiple suspending functional supertypes.

Stable suspend conversions

Kotlin 1.6.0 introduces Stable conversions from regular to suspending functional types. Starting from 1.4.0, the feature supported functional literals and callable references. With 1.6.0, it works with any form of expression. As a call argument, you can now pass any expression of a suitable regular functional type where suspending is expected. The compiler will perform an implicit conversion automatically.

fun getSuspending(suspending: suspend () -> Unit) {} fun suspending() {} fun test(regular: () -> Unit) { getSuspending { } // OK getSuspending(::suspending) // OK getSuspending(regular) // OK }

Stable instantiation of annotation classes

Kotlin 1.5.30 introduced experimental support for instantiation of annotation classes on the JVM platform. With 1.6.0, the feature is available by default both for Kotlin/JVM and Kotlin/JS.

Learn more about instantiation of annotation classes in this KEEP.

Improved type inference for recursive generic types

Kotlin 1.5.30 introduced an improvement to type inference for recursive generic types, which allowed their type arguments to be inferred based only on the upper bounds of the corresponding type parameters. The improvement was available with the compiler option. In version 1.6.0 and later, it is enabled by default.

// Before 1.5.30 val containerA = PostgreSQLContainer<Nothing>(DockerImageName.parse("postgres:13-alpine")).apply { withDatabaseName("db") withUsername("user") withPassword("password") withInitScript("sql/schema.sql") } // With compiler option in 1.5.30 or by default starting with 1.6.0 val containerB = PostgreSQLContainer(DockerImageName.parse("postgres:13-alpine")) .withDatabaseName("db") .withUsername("user") .withPassword("password") .withInitScript("sql/schema.sql")

Changes to builder inference

Builder inference is a type inference flavor which is useful when calling generic builder functions. It can infer the type arguments of a call with the help of type information from calls inside its lambda argument.

We're making multiple changes that are bringing us closer to fully stable builder inference. Starting with 1.6.0:

  • You can make calls returning an instance of a not yet inferred type inside a builder lambda without specifying the -Xunrestricted-builder-inference compiler option introduced in 1.5.30.

  • With -Xenable-builder-inference, you can write your own builders without applying the @BuilderInference annotation.

  • With the -Xenable-builder-inference, builder inference automatically activates if a regular type inference cannot get enough information about a type.

Learn how to write custom generic builders.

Support for annotations on class type parameters

Support for annotations on class type parameters looks like this:

@Target(AnnotationTarget.TYPE_PARAMETER) annotation class BoxContent class Box<@BoxContent T> {}

Annotations on all type parameters are emitted into JVM bytecode so annotation processors are able to use them.

For the motivating use case, read this YouTrack ticket.

Learn more about annotations.

Supporting previous API versions for a longer period

Starting with Kotlin 1.6.0, we will support development for three previous API versions instead of two, along with the current stable one. Currently, we support versions 1.3, 1.4, 1.5, and 1.6.


For Kotlin/JVM, starting with 1.6.0, the compiler can generate classes with a bytecode version corresponding to JVM 17. The new language version also includes optimized delegated properties and repeatable annotations, which we had on the roadmap:

Repeatable annotations with runtime retention for 1.8 JVM target

Java 8 introduced repeatable annotations, which can be applied multiple times to a single code element. The feature requires two declarations to be present in the Java code: the repeatable annotation itself marked with @java.lang.annotation.Repeatable and the containing annotation to hold its values.

Kotlin also has repeatable annotations, but requires only @kotlin.annotation.Repeatable to be present on an annotation declaration to make it repeatable. Before 1.6.0, the feature supported only SOURCE retention and was incompatible with Java's repeatable annotations. Kotlin 1.6.0 removes these limitations. @kotlin.annotation.Repeatable now accepts any retention and makes the annotation repeatable both in Kotlin and Java. Java's repeatable annotations are now also supported from the Kotlin side.

While you can declare a containing annotation, it's not necessary. For example:

  • If an annotation @Tag is marked with @kotlin.annotation.Repeatable, the Kotlin compiler automatically generates a containing annotation class under the name of @Tag.Container:

    @Repeatable annotation class Tag(val name: String) // The compiler generates @Tag.Container containing annotation
  • To set a custom name for a containing annotation, apply the @kotlin.jvm.JvmRepeatable meta-annotation and pass the explicitly declared containing annotation class as an argument:

    @JvmRepeatable(Tags::class) annotation class Tag(val name: String) annotation class Tags(val value: Array<Tag>)

Kotlin reflection now supports both Kotlin's and Java's repeatable annotations via a new function, KAnnotatedElement.findAnnotations().

Learn more about Kotlin repeatable annotations in this KEEP.

Optimize delegated properties which call get/set on the given KProperty instance

We optimized the generated JVM bytecode by omitting the $delegate field and generating immediate access to the referenced property.

For example, in the following code

class Box<T> { private var impl: T = ... var content: T by ::impl }

Kotlin no longer generates the field content$delegate. Property accessors of the content variable invoke the impl variable directly, skipping the delegated property's getValue/setValue operators and thus avoiding the need for the property reference object of the KProperty type.

Thanks to our Google colleagues for the implementation!

Learn more about delegated properties.


Kotlin/Native is receiving multiple improvements and component updates, some of them in the preview state:

Preview of the new memory manager

With Kotlin 1.6.0, you can try the development preview of the new Kotlin/Native memory manager. It moves us closer to eliminating the differences between the JVM and Native platforms to provide a consistent developer experience in multiplatform projects.

One of the notable changes is the lazy initialization of top-level properties, like in Kotlin/JVM. A top-level property gets initialized when a top-level property or function from the same file is accessed for the first time. This mode also includes global interprocedural optimization (enabled only for release binaries), which removes redundant initialization checks.

We've recently published a blog post about the new memory manager. Read it to learn about the current state of the new memory manager and find some demo projects, or jump right to the migration instructions to try it yourself. Please check how the new memory manager works on your projects and share feedback in our issue tracker, YouTrack.

Support for Xcode 13

Kotlin/Native 1.6.0 supports Xcode 13 – the latest version of Xcode. Feel free to update your Xcode and continue working on your Kotlin projects for Apple operating systems.

Compilation of Windows targets on any host

Starting from 1.6.0, you don't need a Windows host to compile the Windows targets mingwX64 and mingwX86. They can be compiled on any host that supports Kotlin/Native.

LLVM and linker updates

We've reworked the LLVM dependency that Kotlin/Native uses under the hood. This brings various benefits, including:

  • Updated LLVM version to 11.1.0.

  • Decreased dependency size. For example, on macOS it's now about 300 MB instead of 1200 MB in the previous version.

  • Excluded dependency on the ncurses5 library that isn't available in modern Linux distributions.

In addition to the LLVM update, Kotlin/Native now uses the LLD linker (a linker from the LLVM project) for MingGW targets. It provides various benefits over the previously used ld.bfd linker, and will allow us to improve runtime performance of produced binaries and support compiler caches for MinGW targets. Note that LLD requires import libraries for DLL linkage. Learn more in this Stack Overflow thread.

Performance improvements

Kotlin/Native 1.6.0 delivers the following performance improvements:

  • Compilation time: compiler caches are enabled by default for linuxX64 and iosArm64 targets. This speeds up most compilations in debug mode (except the first one). Measurements showed about a 200% speed increase on our test projects. The compiler caches have been available for these targets since Kotlin 1.5.0 with additional Gradle properties; you can remove them now.

  • Runtime: iterating over arrays with for loops is now up to 12% faster thanks to optimizations in the produced LLVM code.

Unified compiler plugin ABI with JVM and JS IR backends

In previous versions, authors of compiler plugins had to provide separate artifacts for Kotlin/Native because of the differences in the ABI.

Starting from 1.6.0, the Kotlin Multiplatform Gradle plugin is able to use the embeddable compiler jar – the one used for the JVM and JS IR backends – for Kotlin/Native. This is a step toward unification of the compiler plugin development experience, as you can now use the same compiler plugin artifacts for Native and other supported platforms.

This is a preview version of such support, and it requires an opt-in. To start using generic compiler plugin artifacts for Kotlin/Native, add the following line to gradle.properties: kotlin.native.useEmbeddableCompilerJar=true.

We're planning to use the embeddable compiler jar for Kotlin/Native by default in the future, so it's vital for us to hear how the preview works for you.

If you are an author of a compiler plugin, please try this mode and check if it works for your plugin. Note that depending on your plugin's structure, migration steps may be required. See this YouTrack issue for migration instructions and leave your feedback in the comments.

Detailed error messages for klib linkage failures

The Kotlin/Native compiler now provides detailed error messages for klib linkage errors. The messages now have clear error descriptions, and they also include information about possible causes and ways to fix them.

For example:

  • 1.5.30:

    e: java.lang.IllegalStateException: IrTypeAliasSymbol expected: Unbound public symbol for public kotlinx.coroutines/CancellationException|null[0] <stack trace>
  • 1.6.0:

    e: The symbol of unexpected type encountered during IR deserialization: IrClassPublicSymbolImpl, kotlinx.coroutines/CancellationException|null[0]. IrTypeAliasSymbol is expected. This could happen if there are two libraries, where one library was compiled against the different version of the other library than the one currently used in the project. Please check that the project configuration is correct and has consistent versions of dependencies. The list of libraries that depend on "org.jetbrains.kotlinx:kotlinx-coroutines-core (org.jetbrains.kotlinx:kotlinx-coroutines-core-macosx64)" and may lead to conflicts: <list of libraries and potential version mismatches> Project dependencies: <dependencies tree>

Reworked unhandled exception handling API

We've unified the processing of unhandled exceptions throughout the Kotlin/Native runtime and exposed the default processing as the function processUnhandledException(throwable: Throwable) for use by custom execution environments, like kotlinx.coroutines. This processing is also applied to exceptions that escape operation in Worker.executeAfter(), but only for the new memory manager.

API improvements also affected the hooks that have been set by setUnhandledExceptionHook(). Previously such hooks were reset after the Kotlin/Native runtime called the hook with an unhandled exception, and the program would always terminate right after. Now these hooks may be used more than once, and if you want the program to always terminate on an unhandled exception, either do not set an unhandled exception hook (setUnhandledExceptionHook()), or make sure to call terminateWithUnhandledException() at the end of your hook. This will help you send exceptions to a third-party crash reporting service (like Firebase Crashlytics) and then terminate the program. Exceptions that escape main() and exceptions that cross the interop boundary will always terminate the program, even if the hook did not call terminateWithUnhandledException().


We're continuing to work on stabilizing the IR backend for the Kotlin/JS compiler. Kotlin/JS now has an option to disable downloading of Node.js and Yarn.

Option to use pre-installed Node.js and Yarn

You can now disable downloading Node.js and Yarn when building Kotlin/JS projects and use the instances already installed on the host. This is useful for building on servers without internet connectivity, such as CI servers.

To disable downloading external components, add the following lines to your build.gradle(.kts):

  • Yarn:

    rootProject.plugins.withType<org.jetbrains.kotlin.gradle.targets.js.yarn.YarnPlugin> { rootProject.the<org.jetbrains.kotlin.gradle.targets.js.yarn.YarnRootExtension>().download = false // or true for default behavior }
    rootProject.plugins.withType(org.jetbrains.kotlin.gradle.targets.js.yarn.YarnPlugin) { rootProject.extensions.getByType(org.jetbrains.kotlin.gradle.targets.js.yarn.YarnRootExtension).download = false }
  • Node.js:

    rootProject.plugins.withType<org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsRootPlugin> { rootProject.the<org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsRootExtension>().download = false // or true for default behavior }
    rootProject.plugins.withType(org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsRootPlugin) { rootProject.extensions.getByType(org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsRootExtension).download = false }

Kotlin Gradle plugin

In Kotlin 1.6.0, we changed the deprecation level of the KotlinGradleSubplugin class to 'ERROR'. This class was used for writing compiler plugins. In the following releases, we'll remove this class. Use the class KotlinCompilerPluginSupportPlugin instead.

We removed the kotlin.useFallbackCompilerSearch build option and the noReflect and includeRuntime compiler options. The useIR compiler option has been hidden and will be removed in upcoming releases.

Learn more about the currently supported compiler options in the Kotlin Gradle plugin.

Standard library

The new 1.6.0 version of the standard library stabilizes experimental features, introduces new ones, and unifies its behavior across the platforms:

New readline functions

Kotlin 1.6.0 offers new functions for handling standard input: readln() and readlnOrNull().

Earlier versions

1.6.0 alternative




Reads a line from stdin and returns it, or throws a RuntimeException if EOF has been reached.



Reads a line from stdin and returns it, or returns null if EOF has been reached.

We believe that eliminating the need to use !! when reading a line will improve the experience for newcomers and simplify teaching Kotlin. To make the read-line operation name consistent with its println() counterpart, we've decided to shorten the names of new functions to 'ln'.

println("What is your nickname?") val nickname = readln() println("Hello, $nickname!")
fun main() { //sampleStart var sum = 0 while (true) { val nextLine = readlnOrNull().takeUnless { it.isNullOrEmpty() } ?: break sum += nextLine.toInt() } println(sum) //sampleEnd }

The existing readLine() function will get a lower priority than readln() and readlnOrNull() in your IDE code completion. IDE inspections will also recommend using new functions instead of the legacy readLine().

We're planning to gradually deprecate the readLine() function in future releases.

Stable typeOf()

Version 1.6.0 brings a Stable typeOf() function, closing one of the major roadmap items.

Since 1.3.40, typeOf() was available on the JVM platform as an experimental API. Now you can use it in any Kotlin platform and get KType representation of any Kotlin type that the compiler can infer:

inline fun <reified T> renderType(): String { val type = typeOf<T>() return type.toString() } fun main() { val fromExplicitType = typeOf<Int>() val fromReifiedType = renderType<List<Int>>() }

Stable collection builders

In Kotlin 1.6.0, collection builder functions have been promoted to Stable. Collections returned by collection builders are now serializable in their read-only state.

You can now use buildMap(), buildList(), and buildSet() without the opt-in annotation:

fun main() { //sampleStart val x = listOf('b', 'c') val y = buildList { add('a') addAll(x) add('d') } println(y) // [a, b, c, d] //sampleEnd }

Stable Duration API

The Duration class for representing duration amounts in different time units has been promoted to Stable. In 1.6.0, the Duration API has received the following changes:

  • The first component of the toComponents() function that decomposes the duration into days, hours, minutes, seconds, and nanoseconds now has the Long type instead of Int. Before, if the value didn't fit into the Int range, it was coerced into that range. With the Long type, you can decompose any value in the duration range without cutting off the values that don't fit into Int.

  • The DurationUnit enum is now standalone and not a type alias of java.util.concurrent.TimeUnit on the JVM. We haven't found any convincing cases in which having typealias DurationUnit = TimeUnit could be useful. Also, exposing the TimeUnit API through a type alias might confuse DurationUnit users.

  • In response to community feedback, we're bringing back extension properties like Int.seconds. But we'd like to limit their applicability, so we put them into the companion of the Duration class. While the IDE can still propose extensions in completion and automatically insert an import from the companion, in the future we plan to limit this behavior to cases when the Duration type is expected.

    import kotlin.time.Duration.Companion.seconds fun main() { //sampleStart val duration = 10000 println("There are ${duration.seconds.inWholeMinutes} minutes in $duration seconds") // There are 166 minutes in 10000 seconds //sampleEnd }

    We suggest replacing previously introduced companion functions, such as Duration.seconds(Int), and deprecated top-level extensions like Int.seconds with new extensions in Duration.Companion.

Splitting Regex into a sequence

The Regex.splitToSequence(CharSequence) and CharSequence.splitToSequence(Regex) functions are promoted to Stable. They split the string around matches of the given regex, but return the result as a Sequence so that all operations on this result are executed lazily:

fun main() { //sampleStart val colorsText = "green, red, brown&blue, orange, pink&green" val regex = "[,\\s]+".toRegex() val mixedColor = regex.splitToSequence(colorsText) // or // val mixedColor = colorsText.splitToSequence(regex) .onEach { println(it) } .firstOrNull { it.contains('&') } println(mixedColor) // "brown&blue" //sampleEnd }

Bit rotation operations on integers

In Kotlin 1.6.0, the rotateLeft() and rotateRight() functions for bit manipulations became Stable. The functions rotate the binary representation of the number left or right by a specified number of bits:

fun main() { //sampleStart val number: Short = 0b10001 println(number .rotateRight(2) .toString(radix = 2)) // 100000000000100 println(number .rotateLeft(2) .toString(radix = 2)) // 1000100 //sampleEnd }

Changes for replace() and replaceFirst() in JS

Before Kotlin 1.6.0, the replace() and replaceFirst() Regex functions behaved differently in Java and JS when the replacement string contained a group reference. To make the behavior consistent across all target platforms, we've changed their implementation in JS.

Occurrences of ${name} or $index in the replacement string are substituted with the subsequences corresponding to the captured groups with the specified index or a name:

  • $index – the first digit after '$' is always treated as a part of the group reference. Subsequent digits are incorporated into the index only if they form a valid group reference.Only digits '0'–'9' are considered potential components of the group reference. Note that indexes of captured groups start from '1'. The group with index '0' stands for the whole match.

  • ${name} – the name can consist of Latin letters 'a'–'z', 'A'–'Z', or digits '0'–'9'. The first character must be a letter.

  • To include the succeeding character as a literal in the replacement string, use the backslash character \:

    fun main() { //sampleStart println(Regex("(.+)").replace("Kotlin", """\$ $1""")) // $ Kotlin println(Regex("(.+)").replaceFirst("1.6.0", """\\ $1""")) // \ 1.6.0 //sampleEnd }

    You can use Regex.escapeReplacement() if the replacement string has to be treated as a literal string.

Improvements to the existing API

  • Version 1.6.0 added the infix extension function for Comparable.compareTo(). You can now use the infix form for comparing two objects for order:

    class WrappedText(val text: String) : Comparable<WrappedText> { override fun compareTo(other: WrappedText): Int = this.text compareTo other.text }
  • Regex.replace() in JS is now also not inline to unify its implementation across all platforms.

  • The compareTo() and equals() String functions, as well as the isBlank() CharSequence function now behave in JS exactly the same way they do on the JVM. Previously there were deviations when it came to non-ASCII characters.


In Kotlin 1.6.0, we're starting the deprecation cycle with a warning for some JS-only stdlib API.

concat(), match(), and matches() string functions

  • To concatenate the string with the string representation of a given other object, use plus() instead of concat().

  • To find all occurrences of a regular expression within the input, use findAll() of the Regex class instead of String.match(regex: String).

  • To check if the regular expression matches the entire input, use matches() of the Regex class instead of String.matches(regex: String).

sort() on arrays taking comparison functions

We've deprecated the Array<out T>.sort() function and the inline functions ByteArray.sort(), ShortArray.sort(), IntArray.sort(), LongArray.sort(), FloatArray.sort(), DoubleArray.sort(), and CharArray.sort(), which sorted arrays following the order passed by the comparison function. Use other standard library functions for array sorting.

See the collection ordering section for reference.


Kover – a code coverage tool for Kotlin

With Kotlin 1.6.0, we're introducing Kover – a Gradle plugin for the IntelliJ and JaCoCo Kotlin code coverage agents. It works with all language constructs, including inline functions.

Learn more about Kover on its GitHub repository or in this video:

Coroutines 1.6.0-RC

kotlinx.coroutines 1.6.0-RC is out with multiple features and improvements:

  • Support for the new Kotlin/Native memory manager

  • Introduction of dispatcher views API, which allows limiting parallelism without creating additional threads

  • Migrating from Java 6 to Java 8 target

  • kotlinx-coroutines-test with the new reworked API and multiplatform support

  • Introduction of CopyableThreadContextElement, which gives coroutines a thread-safe write access to ThreadLocal variables

Learn more in the changelog.

Migrating to Kotlin 1.6.0

IntelliJ IDEA and Android Studio will suggest updating the Kotlin plugin to 1.6.0 once it is available.

To migrate existing projects to Kotlin 1.6.0, change the Kotlin version to 1.6.0 and reimport your Gradle or Maven project. Learn how to update to Kotlin 1.6.0.

To start a new project with Kotlin 1.6.0, update the Kotlin plugin and run the Project Wizard from File | New | Project.

The new command-line compiler is available for download on the GitHub release page.

Kotlin 1.6.0 is a feature release and can, therefore, bring changes that are incompatible with your code written for earlier versions of the language. Find the detailed list of such changes in the Compatibility Guide for Kotlin 1.6.

Last modified: 21 May 2024