compatibility-guide-18.md

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 1.7 to Kotlin 1.8.

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

Prohibit the delegation of super calls to an abstract superclass member

Issues: KT-45508, KT-49017, KT-38078

Component: Core language

Incompatible change type: source

Short summary: Kotlin will report a compile error when an explicit or implicit super call is delegated to an abstract member of the superclass, even if there's a default implementation in a super interface

Deprecation cycle:

• 1.5.20: introduce a warning when non-abstract classes that do not override all abstract members are used
• 1.7.0: report a warning if a super call, in fact, accesses an abstract member from a superclass
• 1.7.0: report an error in all affected cases if the -Xjvm-default=all or -Xjvm-default=all-compatibility compatibility modes are enabled; report an error in the progressive mode
• 1.8.0: report an error in cases of declaring a concrete class with a non-overridden abstract method from the superclass, and super calls of Any methods are overridden as abstract in the superclass
• 1.9.0: report an error in all affected cases, including explicit super calls to an abstract method from the super class

Deprecate confusing grammar in when-with-subject

Issue: KT-48385

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.6 deprecated 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, -XXLanguage:-ProhibitConfusingSyntaxInWhenBranches can be used to temporarily revert to the pre-1.8 behavior
• >= 1.9: repurpose some deprecated constructs for new language features

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 is 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
• >= 1.9: fix the downcast behavior in other affected cases

Make private constructors of sealed classes really private

Issue: KT-44866

Component: Core language

Incompatible change type: source

Short summary: after relaxing restrictions on where the inheritors of sealed classes could be declared in the project structure, the default visibility of sealed class constructors became protected. However, until 1.8, Kotlin still allowed calling explicitly declared private constructors of sealed classes outside those classes' scopes

Deprecation cycle:

• 1.6.20: report a warning (or an error in the progressive mode) when a private constructor of a sealed class is called outside that class
• 1.8.0: use default visibility rules for private constructors (a call to a private constructor can be resolved only if this call is inside the corresponding class), the old behavior can be brought back temporarily by specifying the -XXLanguage:-UseConsistentRulesForPrivateConstructorsOfSealedClasses compiler argument

Prohibit using operator == on incompatible numeric types in builder inference context

Issue: KT-45508

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.8 will prohibit using the operator == on incompatible numeric types, for example, Int and Long, in scopes of builder inference lambda functions, the same way as it currently does in other contexts

Deprecation cycle:

• 1.6.20: report a warning (or an error in the progressive mode) when the operator == is used on incompatible numeric types
• 1.8.0: raise the warning to an error, -XXLanguage:-ProperEqualityChecksInBuilderInferenceCalls can be used to temporarily revert to the pre-1.8 behavior

Prohibit if without else and non-exhaustive when in right hand side of elvis operator

Issue: KT-44705

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.8 will prohibit using a non-exhaustive when or the if expression without an else branch on the right hand side of the Elvis operator (?:). Previously, it was allowed if the Elvis operator's result was not used as an expression

Deprecation cycle:

• 1.6.20: report a warning (or an error in the progressive mode) on such non-exhaustive if and when expressions
• 1.8.0: raise this warning to an error,
  -XXLanguage:-ProhibitNonExhaustiveIfInRhsOfElvis can be used to temporarily revert to the pre-1.8 behavior

Prohibit upper bound violation in a generic type alias usage (one type parameter used in several type arguments of the aliased type)

Issues: KT-29168

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.8 will prohibit using a type alias with type arguments that violate the upper bound restrictions of the corresponding type parameters of the aliased type in case when one typealias type parameter is used in several type arguments of the aliased type, for example, typealias Alias<T> = Base<T, T>

Deprecation cycle:

• 1.7.0: report a warning (or an error in the progressive mode) on usages of a type alias with type arguments violating upper bound constraints of the corresponding type parameters of the aliased type
• 1.8.0: raise this warning to an error, -XXLanguage:-ReportMissingUpperBoundsViolatedErrorOnAbbreviationAtSupertypes can be used to temporarily revert to the pre-1.8 behavior

Prohibit upper bound violation in a generic type alias usage (a type parameter used in a generic type argument of a type argument of the aliased type)

Issue: KT-54066

Component: Core language

Incompatible change type: source

Short summary: Kotlin will prohibit using a type alias with type arguments that violate the upper bound restrictions of the corresponding type parameters of the aliased type in case when the typealias type parameter is used as a generic type argument of a type argument of the aliased type, for example, typealias Alias<T> = Base<List<T>>

Deprecation cycle:

• 1.8.0: report a warning when a generic typealias usage has type arguments violating upper bound constraints of the corresponding type parameters of the aliased type
• >=1.10: raise the warning to an error

Prohibit using a type parameter declared for an extension property inside delegate

Issue: KT-24643

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.8 will prohibit delegating extension properties on a generic type to generic types that use the type parameter of the receiver in an unsafe way

Deprecation cycle:

• 1.6.0: report a warning (or an error in the progressive mode) when delegating an extension property to a type that uses type parameters inferred from the delegated property's type arguments in a particular way
• 1.8.0: raise the warning to an error, -XXLanguage:-ForbidUsingExtensionPropertyTypeParameterInDelegate can be used to temporarily revert to the pre-1.8 behavior

Forbid @Synchronized annotation on suspend functions

Issue: KT-48516

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.8 will prohibit placing the @Synchronized annotation on suspend functions because a suspending call should not be allowed to happen inside a synchronized block

Deprecation cycle:

• 1.6.0: report a warning on suspend functions annotated with the @Synchronized annotation, the warning is reported as an error in the progressive mode
• 1.8.0: raise the warning to an error, -XXLanguage:-SynchronizedSuspendError can be used to temporarily revert to the pre-1.8 behavior

Prohibit using spread operator for passing arguments to non-vararg parameters

Issue: KT-48162

Component: Core language

Incompatible change type: source

Short summary: Kotlin allowed passing arrays with the spread operator (*) to non-vararg array parameters in certain conditions. Since Kotlin 1.8, this will be prohibited

Deprecation cycle:

• 1.6.0: report a warning (or an error in the progressive mode) on using the spread operator where a non-vararg array parameter is expected
• 1.8.0: raise the warning to an error, -XXLanguage:-ReportNonVarargSpreadOnGenericCalls can be used to temporarily revert to the pre-1.8 behavior

Prohibit null-safety violation in lambdas passed to functions overloaded by lambda return type

Issue: KT-49658

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.8 will prohibit returning null from lambdas passed to functions overloaded by those lambdas' return types when overloads don't allow a nullable return type. Previously, it was allowed when null was returned from one of the branches of the when operator

Deprecation cycle:

• 1.6.20: report a type mismatch warning (or an error in the progressive mode)
• 1.8.0: raise the warning to an error, -XXLanguage:-DontLoseDiagnosticsDuringOverloadResolutionByReturnType can be used to temporarily revert to the pre-1.8 behavior

Keep nullability when approximating local types in public signatures

Issue: KT-53982

Component: Core language

Incompatible change type: source, binary

Short summary: when a local or anonymous type is returned from an expression-body function without an explicitly specified return type, Kotlin compiler infers (or approximates) the return type using the known supertype of that type. During this, the compiler can infer a non-nullable type where the null value could in fact be returned

Deprecation cycle:

• 1.8.0: approximate flexible types by flexible supertypes
• 1.8.0: report a warning when a declaration is inferred to have a non-nullable type that should be nullable, prompting users to specify the type explicitly
• 1.9.0: approximate nullable types by nullable supertypes, -XXLanguage:-KeepNullabilityWhenApproximatingLocalType can be used to temporarily revert to the pre-1.9 behavior

Do not propagate deprecation through overrides

Issue: KT-47902

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.9 will no longer propagate deprecation from a deprecated member in the superclass to its overriding member in the subclass, thus providing an explicit mechanism for deprecating a member of the superclass while leaving it non-deprecated in the subclass

Deprecation cycle:

• 1.6.20: reporting a warning with the message of the future behavior change and a prompt to either suppress this warning or explicitly write a @Deprecated annotation on an override of a deprecated member
• 1.9.0: stop propagating deprecation status to the overridden members. This change also takes effect immediately in the progressive mode

Prohibit implicit inferring a type variable into an upper bound in the builder inference context

Issue: KT-47986

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.9 will prohibit inferring a type variable into the corresponding type parameter's upper bound in the absence of any use-site type information in the scope of builder inference lambda functions, the same way as it does currently in other contexts

Deprecation cycle:

• 1.7.20: report a warning (or an error in the progressive mode) when a type parameter is inferred into declared upper bounds in the absence of use-site type information
• 1.9.0: raise the warning to an error, -XXLanguage:-ForbidInferringPostponedTypeVariableIntoDeclaredUpperBound can be used to temporarily revert to the pre-1.9 behavior

Prohibit using collection literals in annotation classes anywhere except their parameters declaration

Issue: KT-39041

Component: Core language

Incompatible change type: source

Short summary: Kotlin allows using collection literals in a restricted way - for passing arrays to parameters of annotation classes or specifying default values for these parameters. However besides that, Kotlin allowed using collections literals anywhere else inside an annotation class, for example, in its nested object. Kotlin 1.9 will prohibit using collection literals in annotation classes anywhere except their parameters' default values.

Deprecation cycle:

• 1.7.0: report a warning (or an error in the progressive mode) on array literals in nested objects in annotation classes
• 1.9.0: raise the warning to an error

Prohibit forward referencing of parameters with default values in default value expressions

Issue: KT-25694

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.9 will prohibit forward referencing of parameters with default values in default value expressions of other parameters. This ensures that by the time the parameter is accessed in a default value expression, it would already have a value either passed to the function or initialized by its own default value expression

Deprecation cycle:

• 1.7.0: report a warning (or an error in the progressive mode) when a parameter with default value is references in default value of another parameter that comes before it
• 1.9.0: raise the warning to an error, -XXLanguage:-ProhibitIllegalValueParameterUsageInDefaultArguments can be used to temporarily revert to the pre-1.9 behavior

Prohibit extension calls on inline functional parameters

Issue: KT-52502

Component: Core language

Incompatible change type: source

Short summary: while Kotlin allowed passing an inline functional parameter to another inline function as a receiver, it always resulted in compiler exceptions when compiling such code. Kotlin 1.9 will prohibit this, thus reporting an error instead of crashing the compiler

Deprecation cycle:

• 1.7.20: report a warning (or an error in the progressive mode) for inline extension calls on inline functional parameters
• 1.9.0: raise the warning to an error

Prohibit calls to infix functions named suspend with an anonymous function argument

Issue: KT-49264

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.9 will no longer allow calling infix functions named suspend that have a single argument of a functional type passed as an anonymous function literal

Deprecation cycle:

• 1.7.20: report a warning on suspend infix calls with an anonymous function literal
• 1.9.0: raise the warning to an error, -XXLanguage:-ModifierNonBuiltinSuspendFunError can be used to temporarily revert to the pre-1.9 behavior
• >=1.10: Change how the suspend fun token sequence is interpreted by the parser

Prohibit using captured type parameters in inner classes against their variance

Issue: KT-50947

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.9 will prohibit using type parameters of an outer class having in or out variance in an inner class of that class in positions violating that type parameters' declared variance

Deprecation cycle:

• 1.7.0: report a warning (or an error in the progressive mode) when an outer class' type parameter usage position violates the variance rules of that parameter
• 1.9.0: raise the warning to an error, -XXLanguage:-ReportTypeVarianceConflictOnQualifierArguments can be used to temporarily revert to the pre-1.9 behavior

Prohibit recursive call of a function without explicit return type in compound assignment operators

Issue: KT-48546

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.9 will prohibit calling a function without explicitly specified return type in an argument of a compound assignment operator inside that function's body, as it currently does in other expressions inside the body of that function

Deprecation cycle:

• 1.7.0: report a warning (or an error in the progressive mode) when a function without explicitly specified return type is called recursively in that function's body in a compound assignment operator argument
• 1.9.0: raise the warning to an error

Prohibit unsound calls with expected @NotNull T and given Kotlin generic parameter with nullable bound

Issue: KT-36770

Component: Kotlin/JVM

Incompatible change type: source

Short summary: Kotlin 1.9 will prohibit method calls where a value of a potentially nullable generic type is passed for a @NotNull-annotated parameter of a Java method

Deprecation cycle:

• 1.5.20: report a warning when an unconstrained generic type parameter is passed where a non-nullable type is expected
• 1.9.0: report a type mismatch error instead of the warning above,
  -XXLanguage:-ProhibitUsingNullableTypeParameterAgainstNotNullAnnotated can be used to temporarily revert to the pre-1.8 behavior

Prohibit access to members of a companion of an enum class from entry initializers of this enum

Issue: KT-49110

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.9 will prohibit all kinds of access to the companion object of an enum from an enum entry initializer

Deprecation cycle:

• 1.6.20: report a warning (or an error in the progressive mode) on such companion member access
• 1.9.0: raise the warning to an error, -XXLanguage:-ProhibitAccessToEnumCompanionMembersInEnumConstructorCall can be used to temporarily revert to the pre-1.8 behavior

Deprecate and remove Enum.declaringClass synthetic property

Issue: KT-49653

Component: Kotlin/JVM

Incompatible change type: source

Short summary: Kotlin allowed using the synthetic property declaringClass on Enum values produced from the method getDeclaringClass() of the underlying Java class java.lang.Enum even though this method is not available for Kotlin Enum type. Kotlin 1.9 will prohibit using this property, proposing to migrate to the extension property declaringJavaClass instead

Deprecation cycle:

• 1.7.0: report a warning (or an error in the progressive mode) on declaringClass property usages, propose the migration to declaringJavaClass extension
• 1.9.0: raise the warning to an error, -XXLanguage:-ProhibitEnumDeclaringClass can be used to temporarily revert to the pre-1.9 behavior
• >=1.10: remove declaringClass synthetic property

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 warns about the usage of the 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
• >= 1.9: raise this warning to an error

Standard library

Warn about potential overload resolution change when Range/Progression starts implementing Collection

Issue: KT-49276

Component: Core language / kotlin-stdlib

Incompatible change type: source

Short summary: it is planned to implement the Collection interface in the standard progressions and concrete ranges inherited from them in Kotlin 1.9. This could make a different overload selected in the overload resolution if there are two overloads of some method, one accepting an element and another accepting a collection. Kotlin will make this situation visible by reporting a warning or an error when such overloaded method is called with a range or progression argument

Deprecation cycle:

• 1.6.20: report a warning when an overloaded method is called with the standard progression or its range inheritor as an argument if implementing the Collection interface by this progression/range leads to another overload being selected in this call in future
• 1.8.0: raise this warning to an error
• 1.9.0: stop reporting the error, implement Collection interface in progressions thus changing the overload resolution result in the affected cases

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
• 1.8.20: remove the deprecated functions from stdlib for JS-IR target
• >= 1.9: move the API in kotlinx.* packages to a separate library

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
• 1.9.0: raise the deprecation level to an error
• >=1.10.0: remove the deprecated functions from the public API

Tools

Raise deprecation level of classpath property of KotlinCompile task

Issue: KT-51679

Component: Gradle

Incompatible change type: source

Short summary: the classpath property of the KotlinCompile task is deprecated

Deprecation cycle:

• 1.7.0: the classpath property is deprecated
• 1.8.0: raise the deprecation level to an error
• >=1.9.0: remove the deprecated functions from the public API

Remove kapt.use.worker.api Gradle property

Issue: KT-48827

Component: Gradle

Incompatible change type: behavioral

Short summary: remove 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.compiler.execution.strategy system property

Issue: KT-51831

Component: Gradle

Incompatible change type: behavioral

Short summary: remove the kotlin.compiler.execution.strategy system property used to choose a compiler execution strategy. Use the Gradle property kotlin.compiler.execution.strategy or the compile task property compilerExecutionStrategy instead

Deprecation cycle:

• 1.7.0: raise the deprecation level to a warning
• 1.8.0: remove the property

Changes in compiler options

Issues: KT-27301, KT-48532

Component: Gradle

Incompatible change type: source, binary

Short summary: this change might affect Gradle plugins authors. In kotlin-gradle-plugin, there are additional generic parameters to some internal types (you should add generic types or *). KotlinNativeLink task does not inherit the AbstractKotlinNativeCompile task anymore. KotlinJsCompilerOptions.outputFile and the related KotlinJsOptions.outputFile options are deprecated. Use the Kotlin2JsCompile.outputFileProperty task input instead. The kotlinOptions task input and the kotlinOptions{...} task DSL are in a support mode and will be deprecated in upcoming releases. compilerOptions and kotlinOptions can not be changed on a task execution phase (see one exception in What's new in Kotlin 1.8). freeCompilerArgs returns an immutable List<String> – kotlinOptions.freeCompilerArgs.remove("something") will fail. The useOldBackend property that allowed to use the old JVM backend is removed

Deprecation cycle:

• 1.8.0: KotlinNativeLink task does not inherit the AbstractKotlinNativeCompile. KotlinJsCompilerOptions.outputFile and the related KotlinJsOptions.outputFile options are deprecated. The useOldBackend property that allowed to use the old JVM backend is removed.

Deprecate kotlin.internal.single.build.metrics.file property

Issue: KT-53357

Component: Gradle

Incompatible change type: source

Short summary: deprecate the kotlin.internal.single.build.metrics.file property used to define a single file for build reports. Use the property kotlin.build.report.single_file instead with kotlin.build.report.output=single_file

Deprecation cycle:

• 1.8.0: raise the deprecation level to a warning >= 1.9: delete the property