Compatibility guide for Kotlin 2.0

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 updated changelogs or compiler warnings, this document provides a complete reference for migration from Kotlin 1.9 to Kotlin 2.0.

The Kotlin K2 compiler is introduced as part of Kotlin 2.0. For information on the benefits of the new compiler, changes you might encounter during migration, and how to roll back to the previous compiler, see K2 compiler migration guide.

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

Deprecate use of a synthetic setter on a projected receiver

Issue: KT-54309

Component: Core language

Incompatible change type: source

Short summary: If you use the synthetic setter of a Java class to assign a type that conflicts with the class's projected type, an error is triggered.

Deprecation cycle:

1.8.20: report a warning when a synthetic property setter has a projected parameter type in contravariant position making call-site argument types incompatible

2.0.0: raise the warning to an error

Correct mangling when calling functions with inline class parameters that are overloaded in a Java subclass

Issue: KT-56545

Component: Core language

Incompatible change type: behavioral

Deprecation cycle:

2.0.0: use the correct mangling behavior in function invocations; to revert to the previous behaviour, use the -XXLanguage:-MangleCallsToJavaMethodsWithValueClasses compiler option.

Correct type approximation algorithm for contravariant captured types

Issue: KT-49404

Component: Core language

Incompatible change type: source

Deprecation cycle:

1.8.20: report a warning on problematic calls

2.0.0: raise the warning to an error

Prohibit accessing property value before property initialization

Issue: KT-56408

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: report an error when a property is accessed before initialization in the affected contexts

Report error when there's ambiguity in imported classes with the same name

Issue: KT-57750

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: report an error when resolving a class name that is present in several packages imported with a star import

Generate Kotlin lambdas via invokedynamic and LambdaMetafactory by default

Issue: KT-45375

Component: Core language

Incompatible change type: behavioral

Deprecation cycle:

2.0.0: implement new behavior; lambdas are generated using invokedynamic and LambdaMetafactory by default

Forbid if condition with one branch when an expression is required

Issue: KT-57871

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: report an error in case the if condition has only one branch

Prohibit violation of self upper bounds by passing a star-projection of a generic type

Issue: KT-61718

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: report an error when self upper bounds are violated by passing a star-projection of a generic type

Approximate anonymous types in private inline functions return type

Issue: KT-54862

Component: Core language

Incompatible change type: source

Deprecation cycle:

1.9.0: report a warning on private inline functions if the inferred return type contains an anonymous type

2.0.0: approximate return type of such private inline functions to a supertype

Change overload resolution behavior to prioritize local extension function calls over invoke conventions of local functional type properties

Issue: KT-37592

Component: Core language

Incompatible change type: behavioral

Deprecation cycle:

2.0.0: new overload resolution behavior; function calls are consistently prioritized over invoke conventions

Report error when an inherited member conflict occurs due to a change in a supertype from binary dependency

Issue: KT-51194

Component: Core language

Incompatible change type: source

Deprecation cycle:

1.7.0: report a warning CONFLICTING_INHERITED_MEMBERS_WARNING on declarations where inherited member conflict has occurred in the supertype from binary dependency

2.0.0: raise the warning to an error: CONFLICTING_INHERITED_MEMBERS

Ignore @UnsafeVariance annotations on parameters in invariant types

Issue: KT-57609

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: implement new behavior; the @UnsafeVariance annotation is ignored when reporting errors about type mismatch in contravariant parameters

Change type for out-of-call references to a companion object's member

Issue: KT-54316

Component: Core language

Incompatible change type: source

Deprecation cycle:

1.8.20: report a warning on a companion object function reference type inferred as an unbound reference

2.0.0: change the behavior so that companion object function references are inferred as bound references in all usage contexts

Prohibit exposure of anonymous types from private inline functions

Issue: KT-33917

Component: Core language

Incompatible change type: source

Deprecation cycle:

1.3.0: report a warning on calls to own members of anonymous objects, returned from private inline functions

2.0.0: approximate return type of such private inline functions to a supertype and don't resolve calls to anonymous object members

Report error for an unsound smart cast after a while-loop break

Issue: KT-22379

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: implement new behavior; the old behavior can be restored by switching to language version 1.9

Report error when a variable of an intersection type is assigned a value that is not a subtype of that intersection type

Issue: KT-53752

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: report an error when a variable having an intersection type is assigned a value that is not a subtype of that intersection type

Require opt-in when an interface constructed with a SAM constructor contains a method that requires an opt-in

Issue: KT-52628

Component: Core language

Incompatible change type: source

Deprecation cycle:

1.7.20: report a warning for OptIn usages via SAM constructor

2.0.0: raise the warning to an error for OptIn usages via SAM constructor (or keep reporting the warning if OptIn marker severity is a warning)

Prohibit upper bound violation in typealias constructors

Issue: KT-54066

Component: Core language

Incompatible change type: source

Deprecation cycle:

1.8.0: introduce a warning for cases when the upper bound is violated in typealias constructors

2.0.0: raise the warning to an error in the K2 compiler

Make the real type of a destructuring variable consistent with the explicit type when specified

Issue: KT-57011

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: implement new behavior; the real type of destructuring variable is now consistent with the explicit type when specified

Require opt-in when calling a constructor that has parameter types with default values that require an opt-in

Issue: KT-55111

Component: Core language

Incompatible change type: source

Deprecation cycle:

1.8.20: report a warning on constructor calls that has parameter types which require opt-in

2.0.0: raise the warning to an error (or keep reporting a warning if the OptIn marker severity is a warning)

Report ambiguity between a property and an enum entry with the same name at the same scope level

Issue: KT-52802

Component: Core language

Incompatible change type: source

Deprecation cycle:

1.7.20: report a warning when the compiler resolves to a property instead of an enum entry at the same scope level

2.0.0: report ambiguity when the compiler encounters both a property and an enum entry with the same name at the same scope level in the K2 compiler (leaving the warning as is in the old compiler)

Change qualifier resolution behavior to prefer companion property over enum entry

Issue: KT-47310

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: implement new resolution behavior; companion property is preferred over enum entry

Resolve invoke call receiver type and the invoke function type as if written in desugared form

Issue: KT-58260

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: resolve invoke call receiver type and the invoke function type independently as if they were written in a desugared form

Prohibit exposing private class members through non-private inline functions

Issue: KT-55179

Component: Core language

Incompatible change type: source

Deprecation cycle:

1.9.0: report the PRIVATE_CLASS_MEMBER_FROM_INLINE_WARNING warning when calling private class companion object member from internal inline functions

2.0.0: raise this warning to the PRIVATE_CLASS_MEMBER_FROM_INLINE error

Correct nullability of definitely non-null types in projected generic types

Issue: KT-54663

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: implement new behavior; projected types take into account all in-place not-null types

Change inferred type of prefix increment to match getter's return type instead of inc() operator's return type

Issue: KT-57178

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: implement new behavior; inferred type of prefix increment is changed to match getter's return type instead of the inc() operator's return type

Enforce bound checks when inheriting inner classes from generic inner classes declared in superclasses

Issue: KT-61749

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: report an error when upper bound of the type parameter of a generic inner superclass is violated

Forbid assigning callable references with SAM types when the expected type is a function type with a function type parameter

Issue: KT-64342

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: report a compilation error on callable references with SAM types when the expected type is a function type with a function type parameter

Consider companion object scope for annotation resolution on companion objects

Issue: KT-64299

Component: Core language

Incompatible change type: behavioral

Deprecation cycle:

2.0.0: implement new behavior; the companion object scope is now not ignored during annotation resolution on companion objects

Change evaluation semantics for combination of safe calls and convention operators

Issue: KT-41034

Component: Core language

Incompatible change type: behavioral

Deprecation cycle:

1.4.0: report warnings on each incorrect call

2.0.0: implement new resolution behavior

Require properties with backing field and a custom setter to be immediately initialized

Issue: KT-58589

Component: Core language

Incompatible change type: behavioral

Deprecation cycle:

1.9.20: introduce the MUST_BE_INITIALIZED warning for cases where there is no primary constructor

2.0.0: raise the warning to an error

Prohibit Unit conversion on arbitrary expressions in invoke operator convention call

Issue: KT-61182

Component: Core language

Incompatible change type: behavioral

Deprecation cycle:

2.0.0: report an error when Unit conversion is applied to an arbitrary expression on variable and invoke resolution; use the -XXLanguage:+UnitConversionsOnArbitraryExpressions compiler option to keep the previous behavior with affected expressions.

Forbid nullable assignment to non-null Java field when the field is accessed with a safe call

Issue: KT-62998

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: report to an error in case a nullable is assigned to a non-null Java field

Require star-projected type when overriding a Java method containing a raw-type parameter

Issue: KT-57600

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: implement new behavior; overriding is forbidden for raw type parameters

Change (V)::foo reference resolution when V has a companion

Issue: KT-47313

Component: Core language

Incompatible change type: behavioral

Deprecation cycle:

1.6.0: report a warning on callable references currently bound to companion object instances

2.0.0: implement new behavior; adding parentheses around a type no longer makes it a reference to the type's companion object instance

Forbid implicit non-public API access in effectively public inline functions

Issue: KT-54997

Component: Core language

Incompatible change type: source

Deprecation cycle:

1.8.20: report a compilation warning when implicit non-public API is accessed in public inline functions

2.0.0: raise the warning to an error

Prohibit use-site get annotations on property getters

Issue: KT-57422

Component: Core language

Incompatible change type: source

Deprecation cycle:

1.9.0: report a warning (an error in the progressive mode) on use-site get annotations on getters

2.0.0: raise the warning to the INAPPLICABLE_TARGET_ON_PROPERTY error; use -XXLanguage:-ProhibitUseSiteGetTargetAnnotations to revert to a warning

Prevent implicit inference of type parameters into upper bounds in builder inference lambda functions

Issue: KT-47986

Component: Core language

Incompatible change type: source

Deprecation cycle:

1.7.20: report a warning (or an error in the progressive mode) when the type parameter for a type argument cannot be inferred into declared upper bounds

2.0.0: raise the warning to an error

Keep nullability when approximating local types in public signatures

Issue: KT-53982

Component: Core language

Incompatible change type: source

Deprecation cycle:

1.8.0: flexible types are approximated by flexible supertypes; report a warning when a declaration is inferred to have a non-nullable type that should be nullable, prompting to specify the type explicitly to avoid NPEs

2.0.0: nullable types are approximated by nullable supertypes

Remove special handling for false && ... and false || ... for the purposes of smart-casting

Issue: KT-65776

Component: Core language

Incompatible change type: source

Deprecation cycle:

2.0.0: implement new behavior; no special handling for false && ... and false || ...

Forbid inline open functions in enums

Issue: KT-34372

Component: Core language

Incompatible change type: source

Deprecation cycle:

1.8.0: report a warning on inline open functions in enums

2.0.0: raise the warning to an error

Tools

Visibility changes in Gradle

Issue: KT-64653

Component: Gradle

Incompatible change type: source

Short summary: Previously, certain Kotlin DSL functions and properties intended for a specific DSL context would inadvertently leak into other DSL contexts. We've added the @KotlinGradlePluginDsl annotation, which prevents the exposure of the Kotlin Gradle plugin DSL functions and properties to levels where they are not intended to be available. The following levels are separated from each other:

Kotlin extension

Kotlin target

Kotlin compilation

Kotlin compilation task

Deprecation cycle:

2.0.0: for most popular cases, the compiler reports warnings with suggestions on how to fix them if your build script is configured incorrectly; otherwise, the compiler reports an error

Deprecate kotlinOptions DSL

Issue: KT-63419

Component: Gradle

Incompatible change type: source

Short summary: The ability to configure compiler options through the kotlinOptions DSL and the related KotlinCompile task interface have been deprecated.

Deprecation cycle:

2.0.0: report a warning

Deprecate compilerOptions in KotlinCompilation DSL

Issue: KT-65568

Component: Gradle

Incompatible change type: source

Short summary: The ability to configure the compilerOptions property in the KotlinCompilation DSL has been deprecated.

Deprecation cycle:

2.0.0: report a warning

Deprecate old ways of CInteropProcess handling

Issue: KT-62795

Component: Gradle

Incompatible change type: source

Short summary: the CInteropProcess task and the CInteropSettings class now use the definitionFile property instead of defFile and defFileProperty.

This eliminates the need to add extra dependsOn relations between the CInteropProcess task and the task that generates defFile when the defFile is dynamically generated.

In Kotlin/Native projects, Gradle now lazily verifies the presence of the definitionFile property after the connected task has run later in the build process.

Deprecation cycle:

2.0.0: defFile and defFileProperty parameters are deprecated

Remove kotlin.useK2 Gradle property

Issue: KT-64379

Component: Gradle

Incompatible change type: behavioral

Short summary: The kotlin.useK2 Gradle property has been removed. In Kotlin 1.9.*, it could be used to enable the K2 compiler. In Kotlin 2.0.0 and later, the K2 compiler is enabled by default, so the property has no effect and cannot be used to switch back to the previous compiler.

Deprecation cycle:

1.8.20: the kotlin.useK2 Gradle property is deprecated

2.0.0: the kotlin.useK2 Gradle property is removed

Remove deprecated platform plugin IDs

Issue: KT-65187

Component: Gradle

Incompatible change type: source

Short summary: support for these platform plugin IDs have been removed:

kotlin-platform-android

kotlin-platform-jvm

kotlin-platform-js

org.jetbrains.kotlin.platform.android

org.jetbrains.kotlin.platform.jvm

org.jetbrains.kotlin.platform.js

Deprecation cycle:

1.3: the platform plugin IDs are deprecated

2.0.0: the platform plugin IDs are no longer supported

Remove outputFile JavaScript compiler option

Issue: KT-61116

Component: Gradle

Incompatible change type: source

Short summary: The outputFile JavaScript compiler option has been removed. Instead, you can use the destinationDirectory property of the Kotlin2JsCompile task to specify the directory where the compiled JavaScript output files are written.

Deprecation cycle:

1.9.25: the outputFile compiler option is deprecated

2.0.0: the outputFile compiler option is removed