Compatibility guide for Kotlin 2.1

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Issue: KT-60521

Component: Core language

Incompatible change type: source

Short summary: Kotlin 2.1 introduces language version 2.1 and removes support for language versions 1.4 and 1.5. Language versions 1.6 and 1.7 are deprecated.

Deprecation cycle:

• 1.6.0: report a warning for language version 1.4

• 1.9.0: report a warning for language version 1.5

• 2.1.0: report a warning for language versions 1.6 and 1.7; raise the warning to an error for language versions 1.4 and 1.5

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Issue: KT-70754

Component: Core language

Incompatible change type: behavioral

Short summary: The behavior of the typeOf() function on Kotlin/Native is aligned with Kotlin/JVM to ensure consistency across platforms.

Deprecation cycle:

• 2.1.0: align the typeOf() function behavior on Kotlin/Native

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Issue: KT-69653

Component: Core language

Incompatible change type: source

Short summary: Exposing types with lower visibility through type parameter bounds is now prohibited, addressing inconsistencies in type visibility rules. This change ensures that bounds on type parameters follow the same visibility rules as classes, preventing issues like IR validation errors in the JVM.

Deprecation cycle:

• 2.1.0: report a warning for exposing types via type parameter bounds with lower visibility

• 2.2.0: raise the warning to an error

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Issue: KT-58659

Component: Core language

Incompatible change type: source

Short summary: If a class inherits an abstract var property from an interface and a val property with the same name from a superclass, it now triggers a compilation error. This resolves runtime crashes caused by missing setters in such cases.

Deprecation cycle:

• 2.1.0: report a warning (or an error in progressive mode) when a class inherits an abstract var property from an interface and a val property with the same name from a superclass

• 2.2.0: raise the warning to an error

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Issue: KT-68451

Component: Core language

Incompatible change type: source

Short summary: The compiler now reports an error when uninitialized enum entries are accessed during enum class or entry initialization. This aligns the behavior with member property initialization rules, preventing runtime exceptions and ensuring consistent logic.

Deprecation cycle:

• 2.1.0: report an error when accessing uninitialized enum entries

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Issue: KTLC-34

Component: Core language

Incompatible change type: behavioral

Short summary: The K2 compiler changes its behavior for smart cast propagation by introducing bidirectional propagation of type information for inferred variables, like val x = y. Explicitly typed variables, such as val x: T = y, no longer propagate type information, ensuring stricter adherence to declared types.

Deprecation cycle:

• 2.1.0: enable the new behavior

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Issue: KTLC-35

Component: Core language

Incompatible change type: behavioral

Short summary: The getter for member-extension properties overridden by Java subclasses is now hidden in the subclass's scope, aligning its behavior with that of regular Kotlin properties.

Deprecation cycle:

• 2.1.0: enable the new behavior

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Issue: KTLC-36

Component: Core language

Incompatible change type: binary

Short summary: The visibility of getters and setters for var properties overriding a protected val property is now consistent, with both inheriting the visibility of the overridden val property.

Deprecation cycle:

• 2.1.0: enforce consistent visibility for both getters and setters in K2; K1 remains unaffected

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Issue: KTLC-11

Component: Core language

Incompatible change type: source

Short summary: Nullability mismatches from org.jspecify.annotations, such as @NonNull, @Nullable, and @NullMarked are now treated as errors instead of warnings, enforcing stricter type safety for Java interoperability. To adjust the severity of these diagnostics, use the -Xnullability-annotations compiler option.

Deprecation cycle:

• 1.6.0: report warnings for potential nullability mismatches

• 1.8.20: expand warnings to specific JSpecify annotations, including: @Nullable, @NullnessUnspecified, @NullMarked, and legacy annotations in org.jspecify.nullness (JSpecify 0.2 and earlier)

• 2.0.0: add support for the @NonNull annotation

• 2.1.0: change default mode to strict for JSpecify annotations, converting warnings into errors; use -Xnullability-annotations=@org.jspecify.annotations:warning or -Xnullability-annotations=@org.jspecify.annotations:ignore to override the default behavior

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Issue: KTLC-37

Component: Core language

Incompatible change type: behavioral

Short summary: Overload resolution now consistently prioritizes extension functions over invoke calls in ambiguous cases. This resolves inconsistencies in the resolution logic for local functions and properties. The change applies only after recompilation, without affecting precompiled binaries.

Deprecation cycle:

• 2.1.0: change overload resolution to consistently prioritize extension functions over invoke calls for extension functions with matching signatures; this change applies only after recompilation and does not affect precompiled binaries

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Issue: KTLC-42

Component: Core language

Incompatible change type: source

Short summary: Returning nullable values from lambdas in SAM constructors of JDK function interfaces now triggers a compilation error if the specified type argument is non-nullable. This resolves issues where nullability mismatches could lead to runtime exceptions, ensuring stricter type safety.

Deprecation cycle:

• 2.0.0: report a deprecation warning for nullable return values in SAM constructors of JDK function interfaces

• 2.1.0: enable the new behavior by default

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Issue: KTLC-43

Component: Core language

Incompatible change type: behavioral

Short summary: In Kotlin/Native, private members no longer override or conflict with public members in a superclass, aligning behavior with Kotlin/JVM. This resolves inconsistencies in override resolution and eliminates unexpected behavior caused by separate compilation.

Deprecation cycle:

• 2.1.0: private functions and properties in Kotlin/Native no longer override or affect public members in a superclass, aligning with JVM behavior

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Issue: KTLC-71

Component: Core language

Incompatible change type: source

Short summary: Private operator functions such as getValue(), setValue(), provideDelegate(), hasNext(), and next() can no longer be accessed in public inline functions.

Deprecation cycle:

• 2.0.0: report a deprecation warning for accessing private operator functions in public inline functions

• 2.1.0: raise the warning to an error

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Issue: KTLC-72

Component: Core language

Incompatible change type: source

Short summary: The compiler now ignores @UnsafeVariance annotations during type checks, enforcing stricter type safety for invariant type parameters. This prevents invalid calls that rely on @UnsafeVariance to bypass expected type checks.

Deprecation cycle:

• 2.1.0: activate the new behavior

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Issue: KTLC-100

Component: Core language

Incompatible change type: source

Short summary: The compiler now detects nullability mismatches in Java methods where a warning-level nullable type contains type arguments with stricter, error-level nullability. This ensures that previously ignored errors in type arguments are reported correctly.

Deprecation cycle:

• 2.0.0: report a deprecation warning for nullability mismatches in Java methods with stricter type arguments

• 2.1.0: raise the warning to an error

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Issue: KTLC-3

Component: Core language

Incompatible change type: source

Short summary: The compiler now reports usages of inaccessible types in function literals and type arguments, preventing compilation and runtime failures caused by incomplete type information.

Deprecation cycle:

• 2.0.0: report warnings for function literals with parameters or receivers of inaccessible non-generic types and for types with inaccessible type argument; report errors for function literals with parameters or receivers of inaccessible generic types and for types with inaccessible generic type arguments in specific scenarios

• 2.1.0: raise warnings to errors for function literals with parameters and receivers of inaccessible non-generic types

• 2.2.0: raise warnings to errors for types with inaccessible type arguments

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Issue: KT-43023

Component: kotlin-stdlib

Incompatible change type: source

Short summary: Among other Kotlin standard library APIs, locale-sensitive case conversion functions for Char and String, such as Char.toUpperCase() and String.toLowerCase(), are deprecated. Replace them with locale-agnostic alternatives like String.lowercase() or explicitly specify the locale for locale-sensitive behavior, such as String.lowercase(Locale.getDefault()).

For a comprehensive list of deprecated Kotlin standard library APIs in Kotlin 2.1.0, see KT-71628.

Deprecation cycle:

• 1.4.30: introduce locale-agnostic alternatives as experimental API

• 1.5.0: deprecate locale-sensitive case conversion functions with a warning

• 2.1.0: raise the warning to an error

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Issue: KT-62159

Component: kotlin-stdlib

Incompatible change type: binary

Short summary: The kotlin-stdlib-common.jar artifact, previously used for legacy multiplatform declarations metadata, is deprecated and replaced by .klib files as the standard format for common multiplatform declarations metadata. This change does not affect the main kotlin-stdlib.jar or kotlin-stdlib-all.jar artifacts.

Deprecation cycle:

• 2.1.0: deprecate and remove kotlin-stdlib-common.jar artifact

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Issue: KTLC-27

Component: kotlin-stdlib

Incompatible change type: source

Short summary: StringBuilder.appendln() is deprecated in favor of StringBuilder.appendLine().

Deprecation cycle:

• 1.4.0: the appendln() function is deprecated; report a warning on use

• 2.1.0: raise the warning to an error

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Issue: KT-69545

Component: kotlin-stdlib

Incompatible change type: source

Short summary: Freezing-related APIs in Kotlin/Native, previously marked with the @FreezingIsDeprecated annotation, are now deprecated. This aligns with the introduction of the new memory manager that removes the need for freezing objects for thread sharing. For migration details, see the Kotlin/Native migration guide.

Deprecation cycle:

• 1.7.20: deprecate freezing-related APIs with a warning

• 2.1.0: raise the warning to an error

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Issue: KTLC-23

Component: kotlin-stdlib

Incompatible change type: behavioral

Short summary: Accessing a Map.Entry key-value pair after its associated map has been structurally modified now throws a ConcurrentModificationException.

Deprecation cycle:

• 2.1.0: throw an exception when a map structural modification is detected

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Issue: KT-69255

Component: Gradle

Incompatible change type: source

Short summary: The KotlinCompilationOutput#resourcesDirProvider field is deprecated. Use KotlinSourceSet.resources in your Gradle build script instead to add additional resource directories.

Deprecation cycle:

• 2.1.0: KotlinCompilationOutput#resourcesDirProvider is deprecated

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Issue: KT-69927

Component: Gradle

Incompatible change type: source

Short summary: The registerKotlinJvmCompileTask(taskName, moduleName) function is deprecated in favor of the new registerKotlinJvmCompileTask(taskName, compilerOptions, explicitApiMode) function, which now accepts KotlinJvmCompilerOptions. This allows you to pass a compilerOptions instance, typically from an extension or target, with values used as conventions for the task's options.

Deprecation cycle:

• 2.1.0: the registerKotlinJvmCompileTask(taskName, moduleName) function is deprecated

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Issue: KT-70383

Component: Gradle

Incompatible change type: source

Short summary: The registerKaptGenerateStubsTask(taskName) function is deprecated. Use the new registerKaptGenerateStubsTask(compileTask, kaptExtension, explicitApiMode) function instead. This new version allows you to link values as conventions from the relevant KotlinJvmCompile task, ensuring both tasks are using the same set of options.

Deprecation cycle:

• 2.1.0: the registerKaptGenerateStubsTask(taskName) function is deprecated

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Issue: KT-71602

Component: Gradle

Incompatible change type: behavioral

Short summary: KotlinTopLevelExtension and KotlinTopLevelExtensionConfig interfaces are deprecated in favor of a new KotlinTopLevelExtension interface. This interface merges KotlinTopLevelExtensionConfig, KotlinTopLevelExtension, and KotlinProjectExtension to streamline API hierarchy, and provide official access to the JVM toolchain and compiler properties.

Deprecation cycle:

• 2.1.0: the KotlinTopLevelExtension and KotlinTopLevelExtensionConfig interfaces are deprecated

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Issue: KT-61706

Component: Gradle

Incompatible change type: source

Short summary: The kotlin-compiler-embeddable dependency is removed from the runtime in Kotlin Gradle Plugin (KGP). Required modules are now included directly in KGP artifacts, with the Kotlin language version limited to 2.0 to support compatibility with Gradle Kotlin runtime in versions below 8.2.

Deprecation cycle:

• 2.1.0: report a warning on using kotlin-compiler-embeddable

• 2.2.0: raise the warning to an error

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Issue: KT-70251

Component: Gradle

Incompatible change type: source

Short summary: Compiler module symbols bundled within the Kotlin Gradle Plugin (KGP), such as KotlinCompilerVersion, are hidden from the public API to prevent unintended access in build scripts.

Deprecation cycle:

• 2.1.0: report a warning on accessing these symbols

• 2.2.0: raise the warning to an error

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Issue: KT-68345

Component: Gradle

Incompatible change type: source

Short summary: The stabilityConfigurationFile property in the Compose extension is deprecated in favor of a new stabilityConfigurationFiles property, which allows specifying multiple configuration files.

Deprecation cycle:

• 2.1.0: stabilityConfigurationFile property is deprecated

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Issue: KT-65565

Component: Gradle

Incompatible change type: source

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

• kotlin-platform-common

• org.jetbrains.kotlin.platform.common

Deprecation cycle:

• 1.3: the platform plugin IDs are deprecated

• 2.1.0: the platform plugin IDs are no longer supported