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Keeping the Language Modern and Comfortable Updates are among the fundamental principles in Kotlin Language Design. The former says that constructs which obstruct language evolution should be removed, and the latter says that this removal should be well-communicated beforehand to make code migration as smooth as possible.

While most of the language changes were already announced through other channels, like update changelogs or compiler warnings, this document summarizes them all, providing a complete reference for migration from Kotlin 1.5 to Kotlin 1.6.

Basic terms

In this document we introduce several kinds of compatibility:

  • source: source-incompatible change stops code that used to compile fine (without errors or warnings) from compiling anymore
  • binary: two binary artifacts are said to be binary-compatible if interchanging them doesn't lead to loading or linkage errors
  • behavioral: a change is said to be behavioral-incompatible if the same program demonstrates different behavior before and after applying the change

Remember that those definitions are given only for pure Kotlin. Compatibility of Kotlin code from the other languages perspective (for example, from Java) is out of the scope of this document.

Language

Make when statements with enum, sealed, and Boolean subjects exhaustive by default

Issue: KT-47709

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.6 will warn about the when statement with an enum, sealed, or Boolean subject being non-exhaustive

Deprecation cycle:

  • 1.6.0: introduce a warning when the when statement with an enum, sealed, or Boolean subject is non-exhaustive (error in the progressive mode)
  • 1.7.0: raise this warning to an error

Deprecate confusing grammar in when-with-subject

Issue: KT-48385

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.6 will deprecate several confusing grammar constructs in when condition expressions

Deprecation cycle:

  • 1.6.20: introduce a deprecation warning on the affected expressions
  • 1.8.0: raise this warning to an error
  • >= 1.8: repurpose some deprecated constructs for new language features

Prohibit access to class members in the super constructor call of its companion and nested objects

Issue: KT-25289

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.6 will report an error for arguments of super constructor call of companion and regular objects if the receiver of such arguments refers to the containing declaration

Deprecation cycle:

  • 1.5.20: introduce a warning on the problematic arguments
  • 1.6.0: raise this warning to an error, -XXLanguage:-ProhibitSelfCallsInNestedObjects can be used to temporarily revert to the pre-1.6 behavior

Type nullability enhancement improvements

Issue: KT-48623

Component: Kotlin/JVM

Incompatible change type: source

Short summary: Kotlin 1.7 will change how it loads and interprets type nullability annotations in Java code

Deprecation cycle:

  • 1.4.30: introduce warnings for cases where more precise type nullability could lead to an error
  • 1.7.0: infer more precise nullability of Java types, -XXLanguage:-TypeEnhancementImprovementsInStrictMode can be used to temporarily revert to the pre-1.7 behavior

Prevent implicit coercions between different numeric types

Issue: KT-48645

Component: Kotlin/JVM

Incompatible change type: behavioral

Short summary: Kotlin will avoid converting numeric values automatically to a primitive numeric type where only a downcast to that type was needed semantically

Deprecation cycle:

  • < 1.5.30: the old behavior in all affected cases
  • 1.5.30: fix the downcast behavior in generated property delegate accessors, -Xuse-old-backend can be used to temporarily revert to the pre-1.5.30 fix behavior
  • >= 1.6.20: fix the downcast behavior in other affected cases

Prohibit declarations of repeatable annotation classes whose container annotation violates JLS

Issue: KT-47928

Component: Kotlin/JVM

Incompatible change type: source

Short summary: Kotlin 1.6 will check that the container annotation of a repeatable annotation satisfies the same requirements as in JLS 9.6.3: array-typed value method, retention, and target

Deprecation cycle:

  • 1.5.30: introduce a warning on repeatable container annotation declarations violating JLS requirements (error in the progressive mode)
  • 1.6.0: raise this warning to an error, -XXLanguage:-RepeatableAnnotationContainerConstraints can be used to temporarily disable the error reporting

Prohibit declaring a nested class named Container in a repeatable annotation class

Issue: KT-47971

Component: Kotlin/JVM

Incompatible change type: source

Short summary: Kotlin 1.6 will check that a repeatable annotation declared in Kotlin doesn't have a nested class with the predefined name Container

Deprecation cycle:

  • 1.5.30: introduce a warning on nested classes with the name Container in a Kotlin-repeatable annotation class (error in the progressive mode)
  • 1.6.0: raise this warning to an error, -XXLanguage:-RepeatableAnnotationContainerConstraints can be used to temporarily disable the error reporting

Prohibit @JvmField on a property in the primary constructor that overrides an interface property

Issue: KT-32753

Component: Kotlin/JVM

Incompatible change type: source

Short summary: Kotlin 1.6 will outlaw annotating a property declared in the primary constructor that overrides an interface property with the @JvmField annotation

Deprecation cycle:

  • 1.5.20: introduce a warning on the @JvmField annotation on such properties in the primary constructor
  • 1.6.0: raise this warning to an error, -XXLanguage:-ProhibitJvmFieldOnOverrideFromInterfaceInPrimaryConstructor can be used to temporarily disable the error reporting

Deprecate the enable and the compatibility modes of the compiler option -Xjvm-default

Issue: KT-46329

Component: Kotlin/JVM

Incompatible change type: source

Short summary: Kotlin 1.6.20 will warn about the usage of enable and compatibility modes of the -Xjvm-default compiler option

Deprecation cycle:

  • 1.6.20: introduce a warning on the enable and compatibility modes of the -Xjvm-default compiler option
  • >= 1.8.0: raise this warning to an error

Prohibit super calls from public-abi inline functions

Issue: KT-45379

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.6 will outlaw calling functions with a super qualifier from public or protected inline functions and properties

Deprecation cycle:

  • 1.5.0: introduce a warning on super calls from public or protected inline functions or property accessors
  • 1.6.0: raise this warning to an error, -XXLanguage:-ProhibitSuperCallsFromPublicInline can be used to temporarily disable the error reporting

Prohibit protected constructor calls from public inline functions

Issue: KT-48860

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.6 will outlaw calling protected constructors from public or protected inline functions and properties

Deprecation cycle:

  • 1.4.30: introduce a warning on protected constructor calls from public or protected inline functions or property accessors
  • 1.6.0: raise this warning to an error, -XXLanguage:-ProhibitProtectedConstructorCallFromPublicInline can be used to temporarily disable the error reporting

Prohibit exposing private nested types from private-in-file types

Issue: KT-20094

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.6 will outlaw exposing private nested types and inner classes from private-in-file types

Deprecation cycle:

  • 1.5.0: introduce a warning on private types exposed from private-in-file types
  • 1.6.0: raise this warning to an error, -XXLanguage:-PrivateInFileEffectiveVisibility can be used to temporarily disable the error reporting

Annotation target is not analyzed in several cases for annotations on a type

Issue: KT-28449

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.6 will no longer allow annotations on types that should not be applicable to types

Deprecation cycle:

  • 1.5.20: introduce an error in the progressive mode
  • 1.6.0: introduce an error, -XXLanguage:-ProperCheckAnnotationsTargetInTypeUsePositions can be used to temporarily disable the error reporting

Prohibit calls to functions named suspend with a trailing lambda

Issue: KT-22562

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.6 will no longer allow calling functions named suspend that have the single argument of a functional type passed as a trailing lambda

Deprecation cycle:

  • 1.3.0: introduce a warning on such function calls
  • 1.6.0: raise this warning to an error
  • >= 1.7.0: introduce changes to the language grammar, so that suspend before { is parsed as a keyword

标准库

Remove brittle contains optimization in minus/removeAll/retainAll

Issue: KT-45438

Component: kotlin-stdlib

Incompatible change type: behavioral

Short summary: Kotlin 1.6 will no longer perform conversion to set for the argument of functions and operators that remove several elements from collection/iterable/array/sequence.

Deprecation cycle:

  • < 1.6: the old behavior: the argument is converted to set in some cases
  • 1.6.0: if the function argument is a collection, it's no longer converted to Set. If it's not a collection, it can be converted to List instead.
    The old behavior can be temporarily turned back on JVM by setting the system property kotlin.collections.convert_arg_to_set_in_removeAll=true
  • >= 1.7: the system property above will no longer have an effect

Change value generation algorithm in Random.nextLong

Issue: KT-47304

Component: kotlin-stdlib

Incompatible change type: behavioral

Short summary: Kotlin 1.6 changes the value generation algorithm in the Random.nextLong function to avoid producing values out of the specified range.

Deprecation cycle:

  • 1.6.0: the behavior is fixed immediately

Gradually change the return type of collection min and max functions to non-nullable

Issue: KT-38854

Component: kotlin-stdlib

Incompatible change type: source

Short summary: the return type of collection min and max functions will be changed to non-nullable in Kotlin 1.7

Deprecation cycle:

  • 1.4.0: introduce ...OrNull functions as synonyms and deprecate the affected API (see details in the issue)
  • 1.5.0: raise the deprecation level of the affected API to an error
  • 1.6.0: hide the deprecated functions from the public API
  • >= 1.7: reintroduce the affected API but with non-nullable return type

Deprecate floating-point array functions: contains, indexOf, lastIndexOf

Issue: KT-28753

Component: kotlin-stdlib

Incompatible change type: source

Short summary: Kotlin deprecates floating-point array functions contains, indexOf, lastIndexOf that compare values using the IEEE-754 order instead of the total order

Deprecation cycle:

  • 1.4.0: deprecate the affected functions with a warning
  • 1.6.0: raise the deprecation level to an error
  • >= 1.7: hide the deprecated functions from the public API

Migrate declarations from kotlin.dom and kotlin.browser packages to kotlinx.*

Issue: KT-39330

Component: kotlin-stdlib (JS)

Incompatible change type: source

Short summary: declarations from the kotlin.dom and kotlin.browser packages are moved to the corresponding kotlinx.* packages to prepare for extracting them from stdlib

Deprecation cycle:

  • 1.4.0: introduce the replacement API in kotlinx.dom and kotlinx.browser packages
  • 1.4.0: deprecate the API in kotlin.dom and kotlin.browser packages and propose the new API above as a replacement
  • 1.6.0: raise the deprecation level to an error
  • >= 1.7: remove the deprecated functions from stdlib
  • >= 1.7: move the API in kotlinx.* packages to a separate library

Make Regex.replace function not inline in Kotlin/JS

Issue: KT-27738

Component: kotlin-stdlib (JS)

Incompatible change type: source

Short summary: the Regex.replace function with the functional transform parameter will no longer be inline in Kotlin/JS

Deprecation cycle:

  • 1.6.0: remove the inline modifier from the affected function

Different behavior of the Regex.replace function in JVM and JS when replacement string contains group reference

Issue: KT-28378

Component: kotlin-stdlib (JS)

Incompatible change type: behavioral

Short summary: the function Regex.replace in Kotlin/JS with the replacement pattern string will follow the same syntax of that pattern as in Kotlin/JVM

Deprecation cycle:

  • 1.6.0: change the replacement pattern handling in Regex.replace of the Kotlin/JS stdlib

Use the Unicode case folding in JS Regex

Issue: KT-45928

Component: kotlin-stdlib (JS)

Incompatible change type: behavioral

Short summary: the Regex class in Kotlin/JS will use unicode flag when calling the underlying JS Regular expressions engine to search and compare characters according to the Unicode rules. This brings certain version requirements of the JS environment and causes more strict validation of unnecessary escaping in the regex pattern string.

Deprecation cycle:

  • 1.5.0: enable the Unicode case folding in most functions of the JS Regex class
  • 1.6.0: enable the Unicode case folding in the Regex.replaceFirst function

Deprecate some JS-only API

Issue: KT-48587

Component: kotlin-stdlib (JS)

Incompatible change type: source

Short summary: a number of JS-only functions in stdlib are deprecated for removal. They include: String.concat(String), String.match(regex: String), String.matches(regex: String), and the sort functions on arrays taking a comparison function, for example, Array<out T>.sort(comparison: (a: T, b: T) -> Int)

Deprecation cycle:

  • 1.6.0: deprecate the affected functions with a warning
  • 1.7.0: raise the deprecation level to an error
  • 1.8.0: remove the deprecated functions from the public API

Hide implementation- and interop-specific functions from the public API of classes in Kotlin/JS

Issue: KT-48587

Component: kotlin-stdlib (JS)

Incompatible change type: source, binary

Short summary: the functions HashMap.createEntrySet and AbstactMutableCollection.toJSON change their visibility to internal

Deprecation cycle:

  • 1.6.0: make the functions internal, thus removing them from the public API

Tools

Deprecate KotlinGradleSubplugin class

Issue: KT-48830

Component: Gradle

Incompatible change type: source

Short summary: the class KotlinGradleSubplugin will be deprecated in favor of KotlinCompilerPluginSupportPlugin

Deprecation cycle:

  • 1.6.0: raise the deprecation level to an error
  • >= 1.7.0: remove the deprecated class

Remove kotlin.useFallbackCompilerSearch build option

Issue: KT-46719

Component: Gradle

Incompatible change type: source

Short summary: remove the deprecated 'kotlin.useFallbackCompilerSearch' build option

Deprecation cycle:

  • 1.5.0: raise the deprecation level to a warning
  • 1.6.0: remove the deprecated option

Remove several compiler options

Issue: KT-48847

Component: Gradle

Incompatible change type: source

Short summary: remove the deprecated noReflect and includeRuntime compiler options

Deprecation cycle:

  • 1.5.0: raise the deprecation level to an error
  • 1.6.0: remove the deprecated options

Deprecate useIR compiler option

Issue: KT-48847

Component: Gradle

Incompatible change type: source

Short summary: hide the deprecated useIR compiler option

Deprecation cycle:

  • 1.5.0: raise the deprecation level to a warning
  • 1.6.0: hide the option
  • >= 1.7.0: remove the deprecated option

Deprecate kapt.use.worker.api Gradle property

Issue: KT-48826

Component: Gradle

Incompatible change type: source

Short summary: deprecate the kapt.use.worker.api property that allowed to run kapt via Gradle Workers API (default: true)

Deprecation cycle:

  • 1.6.20: raise the deprecation level to a warning
  • >= 1.8.0: remove this property

Remove kotlin.parallel.tasks.in.project Gradle property

Issue: KT-46406

Component: Gradle

Incompatible change type: source

Short summary: remove the kotlin.parallel.tasks.in.project property

Deprecation cycle:

  • 1.5.20: raise the deprecation level to a warning
  • 1.6.20: remove this property

Deprecate kotlin.experimental.coroutines Gradle DSL option and kotlin.coroutines Gradle property

Issue: KT-50369

Component: Gradle

Incompatible change type: source

Short summary: deprecate the kotlin.experimental.coroutines Gradle DSL option and the kotlin.coroutines property

Deprecation cycle:

  • 1.6.20: raise the deprecation level to a warning
  • >= 1.7.0: remove the DSL option and the property