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‎docs/topics/compatibility-guides/compatibility-guide-23.md‎

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## Language
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### Remove language versions 1.8 and 1.9
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### Drop support in `-language-version` for 1.8 and 1.9
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> **Issue**: [KT-76343](https://youtrack.jetbrains.com/issue/KT-76343), [KT-76344](https://youtrack.jetbrains.com/issue/KT-76344).
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>
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> **Component**: Core language
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> **Component**: Compiler
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> **Incompatible change type**: source
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>
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> **Short summary**: Kotlin 2.3 introduces language version 2.3 and removes support for language version 1.8.
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> Support for language version 1.9 is also removed for non-JVM platforms. Language versions 1.8 and 1.9 are deprecated.
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> **Short summary**: Starting with Kotlin 2.3, the compiler no longer supports [`-language-version=1.8`](compiler-reference.md#language-version-version).
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> Support for `-language-version=1.9` is also removed for non-JVM platforms.
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>
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> **Deprecation cycle**:
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>
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> - 2.2.0: report a warning for language versions 1.8 and 1.9
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> - 2.2.0: report a warning when using `-language-version` with versions 1.8 and 1.9.
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> - 2.3.0: raise the warning to an error for language version 1.8 on all platforms and for language version 1.9 on non-JVM platforms.
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### Enable invokedynamic for annotated lambdas by default
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### Handle `UPPER_BOUND_VIOLATED` on implicit type arguments
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> **Issue**: [KTLC-287](https://youtrack.jetbrains.com/issue/KTLC-287)
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>
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> **Component**: Core language
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>
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> **Incompatible change type**: behavioral
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>
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> **Short summary**: Previously, the compiler never reported `UPPER_BOUND_VIOLATED` for implicit type parameters. This
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> has been fixed in Kotlin 2.3.0 so that `UPPER_BOUND_VIOLATED` is reported consistently across all type parameters.
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>
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> **Deprecation cycle**:
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>
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> - 2.2.20: report deprecation warnings for bounds violations by implicit type arguments
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> - 2.3.0: raise the warning to an error for `UPPER_BOUND_VIOLATED` on implicit type arguments
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### Prohibit `@JvmSerializableLambda` annotation on `inline` and `crossinline` lambdas
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> **Issue**: [KTLC-9](https://youtrack.jetbrains.com/issue/KTLC-9)
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>
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> **Component**: Core language
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>
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> **Incompatible change type**: behavioral
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>
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> **Short summary**: Prohibit adding the `@JvmSerializableLambda` annotation on `inline` and `crossinline` lambdas
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> because it has no effect. `inline` and `crossinline` lambdas are not serializable.
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>
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> **Deprecation cycle**:
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>
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> - 2.1.20: report a warning when the `@JvmSerializableLambda` is applied to `inline` and `crossinline` lambdas
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> - 2.3.0: raise the warning to an error; this change can be enabled in progressive mode
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### Prohibit delegating a Kotlin interface to a Java class when the generic signatures don't match
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> **Issue**: [KTLC-267](https://youtrack.jetbrains.com/issue/KTLC-267)
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>
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> **Component**: Core language
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>
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> **Incompatible change type**: behavioral
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>
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> **Short summary**: Kotlin 2.3.0 forbids delegation to a Java class that implements a generic interface method with
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> a non-generic override. Previously, allowing this behavior led to type mismatches and `ClassCastException` reported at runtime.
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> This change shifts the error from runtime to compile time.
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>
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> **Deprecation cycle**:
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>
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> - 2.1.20: report a warning
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> - 2.3.0: raise the warning to an error
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### Deprecate unintended use of `return` in expression-bodied functions without explicit return type
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> **Issue**: [KTLC-288](https://youtrack.jetbrains.com/issue/KTLC-288)
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>
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> **Component**: Core language
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>
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> **Incompatible change type**: behavioral
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>
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> **Short summary**: Kotlin now deprecates using `return` inside expression bodies when the function's return type is not
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> explicitly declared.
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>
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> **Deprecation cycle**:
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>
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> - 2.3.0: report a warning
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> - 2.4.0: raise the warning to an error
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### Prohibit inheritance from nullable supertypes introduced via typealias
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> **Issue**: [KTLC-279](https://youtrack.jetbrains.com/issue/KTLC-279)
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>
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> **Component**: Core language
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>
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> **Incompatible change type**: behavioral
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>
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> **Short summary**: Kotlin now reports an error when attempting to inherit from a nullable typealias, consistent with
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> how it already handles direct nullable supertypes.
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>
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> **Deprecation cycle**:
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>
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> - 2.2.0: report a warning
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> - 2.3.0: raise the warning to an error
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### Unify generic signature generation for top-level lambdas and call arguments
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> **Issue**: [KTLC-277](https://youtrack.jetbrains.com/issue/KTLC-277)
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>
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> **Component**: Core language
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>
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> **Incompatible change type**: behavioral
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>
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> **Short summary**: Kotlin 2.3 uses the same type-checking logic for top-level lambdas as it does for lambdas passed
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> as call arguments, ensuring consistent generic signature generation across both cases.
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>
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> **Deprecation cycle**:
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>
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> - 2.3.0: introduce new behavior; not applicable for progressive mode
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### Prohibit reified type parameters from being inferred as intersection types
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> **Issue**: [KTLC-13](https://youtrack.jetbrains.com/issue/KTLC-13)
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>
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> **Component**: Core language
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>
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> **Incompatible change type**: behavioral
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> **Short summary**: Kotlin 2.3 prohibits situations where a reified type parameter is inferred to an intersection type,
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> due to the risk of incorrect runtime behavior.
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>
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> **Deprecation cycle**:
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>
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> - 2.1.0: report a warning when a reified type parameter is inferred to an intersection type
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> - 2.3.0: raise the warning to an error
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### Prohibit exposing less-visible types through type parameter bounds
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> **Issue**: [KTLC-275](https://youtrack.jetbrains.com/issue/KTLC-275)
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> **Component**: Core language
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>
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> **Incompatible change type**: behavioral
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> **Short summary**: Kotlin 2.3 forbids using type parameter bounds that expose types with more restrictive visibility
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> than the function or declaration itself, aligning the rules for functions with those already applied to classes.
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>
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> **Deprecation cycle**:
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>
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> - 2.1.0: report a warning on the problematic type parameter bound
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> - 2.3.0: raise the warning to an error
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### Deprecate misleading Char-to-number conversions and introduce explicit digit and code APIs
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> **Issue**: [KTLC-278](https://youtrack.jetbrains.com/issue/KTLC-278)
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> **Issue**: [KTLC-321](https://youtrack.jetbrains.com/issue/KTLC-321)
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> **Component**: Core language
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> **Incompatible change type**: behavioral
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> **Short summary**: Lambdas with annotations now use `invokedynamic` through `LambdaMetafactory` by default, aligning their behavior with Java lambdas.
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> This affects reflection-based code that relied on retrieving annotations from generated lambda classes.
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> To revert to the old behavior, use the `-Xindy-allow-annotated-lambdas=false` compiler option.
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> **Short summary**: Kotlin 2.3 deprecates `Char.toX()` and `X.toChar()` conversions for numeric types and introduces new,
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> explicit APIs for accessing a character's code and digit value.
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> **Deprecation cycle**:
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> - 2.2.0: enable `invokedynamic` for annotated lambdas by default
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> - 1.4.30: introduce new functions as Experimental
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> - 1.5.0: promote the new functions to Stable; report warnings for old functions with suggestions for replacements
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> - 2.3.0: raise the warnings to errors
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## Standard library
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### Deprecate `String.subSequence(start, end)` function
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> **Issue**: [KT-74493](https://youtrack.jetbrains.com/issue/KT-74493)
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> **Issue**: [KTLC-282](https://youtrack.jetbrains.com/issue/KTLC-282)
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> **Component**: kotlin-stdlib
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### Deprecate `kotlin.io.createTempDirectory()` and `kotlin.io.createTempFile()` functions
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> **Issue**: [KT-81078](https://youtrack.jetbrains.com/issue/KT-81078)
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> **Issue**: [KTLC-281](https://youtrack.jetbrains.com/issue/KTLC-281)
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> **Component**: kotlin-stdlib
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### Hide `InputStream.readBytes(Int)` function
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> **Issue**: [KT-79192](https://youtrack.jetbrains.com/issue/KT-79192)
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> **Issue**: [KTLC-280](https://youtrack.jetbrains.com/issue/KTLC-280)
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> **Component**: kotlin-stdlib
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> **Incompatible change type**: behavioral
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> **Short summary**: The [`Iterable<T>.intersect()`](https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.collections/intersect.html) and [`Iterable<T>.subtract()`](https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.collections/subtract.html) functions now test membership for each
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> receiver element before adding it to the result set.
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> The result set compares elements using `Any::equals`,
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> receiver element before adding it to the result set. The result set compares elements using `Any::equals`,
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> ensuring correct results even when the argument collection uses referential equality (for example, `IdentityHashMap.keys˙).
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> **Deprecation cycle**:

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