Compatibility Guide for Kotlin 1.3

Keeping the Language Modern and Comfortable Updates are among the fundamental principles in Kotlin Language Design. The former says that constructions 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 1.2 to Kotlin 1.3

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 one and the same program demonstrates different behavior before and after applying the change

One has to remember that those definitions are given only for pure Kotlin. Compatibility of Kotlin code from the other languages perspective (e.g. from Java) is out of the scope of this document.

Evaluation order of constructor arguments regarding <clinit> call

Issue: KT-19532

Component: Kotlin/JVM

Incompatible change type: behavioral

Short summary: evaluation order with respect to class initialization is changed in 1.3

Deprecation cycle:

• <1.3: old behavior (see details in the Issue)
• >= 1.3: behavior changed, -Xnormalize-constructor-calls=disable can be used to temporarily revert to pre-1.3 behavior. Support for this flag is going to be removed in the next major release.

Missing getter-targeted annotations on annotation constructor parameters

Issue: KT-25287

Component: Kotlin/JVM

Incompatible change type: behavioral

Short summary: getter-target annotations on annotations constructor parameters will be properly written to classfiles in 1.3

Deprecation cycle:

• <1.3: getter-target annotations on annotation constructor parameters are not applied
• >=1.3: getter-target annotations on annotation constructor parameters are properly applied and written to the generated code

Missing errors in class constructor’s @get: annotations

Issue: KT-19628

Component: Core language

Incompatible change type: Source

Short summary: errors in getter-target annotations will be reported properly in 1.3

Deprecation cycle:

• <1.2: compilation errors in getter-target annotations were not reported, causing incorrect code to be compiled fine.
• 1.2.x: errors reported only by tooling, the compiler still compiles such code without any warnings
• >=1.3: errors reported by the compiler too, causing erroneous code to be rejected

Nullability assertions on access to Java types annotated with @NotNull

Issue: KT-20830

Component: Kotlin/JVM

Incompatible change type: Behavioral

Short summary: nullability assertions for Java-types annotated with not-null annotations will be generated more aggressively, causing code which passes null here to fail faster.

Deprecation cycle:

• <1.3: the compiler could miss such assertions when type inference was involved, allowing potential null propagation during compilation against binaries (see Issue for details).
• >=1.3: the compiler generates missed assertions. This can case code which was (erroneously) passing nulls here fail faster.
  -XXLanguage:-StrictJavaNullabilityAssertions can be used to temporarily return to the pre-1.3 behavior. Support for this flag will be removed in the next major release.

Unsound smartcasts on enum members

Issue: KT-20772

Component: Core language

Incompatible change type: Source

Short summary: a smartcast on a member of one enum entry will be correctly applied to only this enum entry

Deprecation cycle:

• <1.3: a smartcast on a member of one enum entry could lead to an unsound smartcast on the same member of other enum entries.
• >=1.3: smartcast will be properly applied only to the member of one enum entry.
  -XXLanguage:-SoundSmartcastForEnumEntries will temporarily return old behavior. Support for this flag will be removed in the next major release.

val backing field reassignment in getter

Issue: KT-16681

Components: Core language

Incompatible change type: Source

Short summary: reassignment of the backing field of val-property in its getter is now prohibited

Deprecation cycle:

• <1.2: Kotlin compiler allowed to modify backing field of val in its getter. Not only it violates Kotlin semantic, but also generates ill-behaved JVM bytecode which reassigns final field.
• 1.2.X: deprecation warning is reported on code which reassigns backing field of val
• >=1.3: deprecation warnings are elevated to errors

Array capturing before the for-loop where it is iterated

Issue: KT-21354

Component: Kotlin/JVM

Incompatible change type: Source

Short summary: if an expression in for-loop range is a local variable updated in a loop body, this change affects loop execution. This is inconsistent with iterating over other containers, such as ranges, character sequences, and collections.

Deprecation cycle:

• <1.2: described code patterns are compiled fine, but updates to local variable affect loop execution
• 1.2.X: deprecation warning reported if a range expression in a for-loop is an array-typed local variable which is assigned in a loop body
• 1.3: change behavior in such cases to be consistent with other containers

Nested classifiers in enum entries

Issue: KT-16310

Component: Core language

Incompatible change type: Source

Short summary: since Kotlin 1.3, nested classifiers (classes, object, interfaces, annotation classes, enum classes) in enum entries are prohibited

Deprecation cycle:

• <1.2: nested classifiers in enum entries are compiled fine, but may fail with exception at runtime
• 1.2.X: deprecation warnings reported on the nested classifiers
• >=1.3: deprecation warnings elevated to errors

Data class overriding copy

Issue: KT-19618

Components: Core language

Incompatible change type: Source

Short summary: since Kotlin 1.3, data classes are prohibited to override copy()

Deprecation cycle:

• <1.2: data classes overriding copy() are compiled fine but may fail at runtime/expose strange behavior
• 1.2.X: deprecation warnings reported on data classes overriding copy()
• >=1.3: deprecation warnings elevated to errors

Inner classes inheriting Throwable that capture generic parameters from the outer class

Issue: KT-17981

Component: Core language

Incompatible change type: Source

Short summary: since Kotlin 1.3, inner classes are not allowed to inherit Throwable

Deprecation cycle:

• <1.2: inner classes inheriting Throwable are compiled fine. If such inner classes happen to capture generic parameters, it could lead to strange code patterns which fail at runtime.
• 1.2.X: deprecation warnings reported on inner classes inheriting Throwable
• >=1.3: deprecation warnings elevated to errors

Visibility rules regarding complex class hierarchies with companion objects

Issues: KT-21515, KT-25333

Component: Core language

Incompatible change type: Source

Short summary: since Kotlin 1.3, rules of visibility by short names are stricter for complex class hierarchies involving companion objects and nested classifiers.

Deprecation cycle:

• <1.2: old visibility rules (see Issue for details)
• 1.2.X: deprecation warnings reported on short names which are not going to be accessible anymore. Tooling suggests automated migration by adding full name.
• >=1.3: deprecation warnings elevated to errors. Offending code should add full qualifiers or explicit imports

Non-constant vararg annotation parameters

Issue: KT-23153

Component: Core language

Incompatible change type: Source

Short summary: since Kotlin 1.3, setting non-constant values as vararg annotation parameters is prohibited

Deprecation cycle:

• <1.2: the compiler allows to pass non-constant value for vararg annotation parameter, but actually drops that value during bytecode generation, leading to non-obvious behavior
• 1.2.X: deprecation warnings reported on such code patterns
• >=1.3: deprecation warnings elevated to errors

Local annotation classes

Issue: KT-23277

Component: Core language

Incompatible change type: Source

Short summary: since Kotlin 1.3 local annotation classes are not supported

Deprecation cycle:

• <1.2: the compiler compiled local annotation classes fine
• 1.2.X: deprecation warnings reported on local annotation classes
• >=1.3: deprecation warnings elevated to errors

Smartcasts on local delegated properties

Issue: KT-22517

Component: Core language

Incompatible change type: Source

Short summary: since Kotlin 1.3 smartcasts on local delegated properties are not allowed

Deprecation cycle:

• <1.2: the compiler allowed to smartcast local delegated property, which could lead to unsound smartcast in case of ill-behaved delegates
• 1.2.X: smartcasts on local delegated properries are reported as deprecated (the compiler issues warnings)
• >=1.3: deprecation warnings elevated to errors

mod operator convention

Issues: KT-24197

Component: Core language

Incompatible change type: Source

Short summary: since Kotlin 1.3 declaration of mod operator is prohibited, as well as calls which resolve to such declarations

Deprecation cycle:

• 1.1.X, 1.2.X: report warnings on declarations of operator mod, as well as on calls which resolve to it
• 1.3.X: elevate warnings to error, but still allow to resolve to operator mod declarations
• 1.4.X: do not resolve calls to operator mod anymore

Passing single element to vararg in named form

Issues: KT-20588, KT-20589. See also KT-20171

Component: Core language

Incompatible change type: Source

Short summary: in Kotlin 1.3, assigning single element to vararg is deprecated and should be replaced with consecutive spread and array construction.

Deprecation cycle:

• <1.2: assigning one value element to vararg in named form compiles fine and is treated as assigning single element to array, causing non-obvious behavior when assigning array to vararg
• 1.2.X: deprecation warnings are reported on such assignments, users are suggested to switch to consecutive spread and array construction.
• 1.3.X: warnings are elevated to errors
• >= 1.4: change semantic of assigning single element to vararg, making assignment of array equivalent to the assignment of a spread of an array

Retention of annotations with target EXPRESSION

Issue: KT-13762

Component: Core language

Incompatible change type: Source

Short summary: since Kotlin 1.3, only SOURCE retention is allowed for annotations with target EXPRESSION

Deprecation cycle:

• <1.2: annotations with target EXPRESSION and retention other than SOURCE are allowed, but silently ignored at use-sites
• 1.2.X: deprecation warnings are reported on declarations of such annotations
• >=1.3: warnings are elevated to errors

Annotations with target PARAMETER shouldn't be applicable to parameter's type

Issue: KT-9580

Component: Core language

Incompatible change type: Source

Short summary: since Kotlin 1.3, error about wrong annotation target will be properly reported when annotation with target PARAMETER is applied to parameter's type

Deprecation cycle:

• <1.2: aforementioned code patterns are compiled fine; annotations are silently ignored and not present in the bytecode
• 1.2.X: deprecation warnings are reported on such usages
• >=1.3: warnings are elevated to errors

Array.copyOfRange throws an exception when indices are out of bounds instead of enlarging the returned array

Issue: KT-19489

Component: kotlin-stdlib (JVM)

Incompatible change type: Behavioral

Short summary: since Kotlin 1.3, ensure that the toIndex argument of Array.copyOfRange, which represents the exclusive end of the range being copied, is not greater than the array size and throw IllegalArgumentException if it is.

Deprecation cycle:

• <1.3: in case toIndex in the invocation of Array.copyOfRange is greater than the array size, the missing elements in range fill be filled with nulls, violating soundness of the Kotlin type system.
• >=1.3: check that toIndex is in the array bounds, and throw exception if it isn't

Progressions of ints and longs with a step of Int.MIN_VALUE and Long.MIN_VALUE are outlawed and won’t be allowed to be instantiated

Issue: KT-17176

Component: kotlin-stdlib (JVM)

Incompatible change type: Behavioral

Short summary: since Kotlin 1.3, prohibit step value for integer progressions being the minimum negative value of its integer type (Long or Int), so that calling IntProgression.fromClosedRange(0, 1, step = Int.MIN_VALUE) will throw IllegalArgumentException

Deprecation cycle:

• <1.3: it was possible to create an IntProgression with Int.MIN_VALUE step, which yields two values [0, -2147483648], which is non-obvious behavior
• >=1.3: throw IllegalArgumentException if the step is the minimum negative value of its integer type

Check for index overflow in operations on very long sequences

Issue: KT-16097

Component: kotlin-stdlib (JVM)

Incompatible change type: Behavioral

Short summary: since Kotlin 1.3, make sure index, count and similar methods do not overflow for long sequences. See the Issue for the full list of affected methods.

Deprecation cycle:

• <1.3: calling such methods on very long sequences could produce negative results due to integer overflow
• >=1.3: detect overflow in such methods and throw exception immediately

Unify split by an empty match regex result across the platforms

Issue: KT-21049

Component: kotlin-stdlib (JVM)

Incompatible change type: Behavioral

Short summary: since Kotlin 1.3, unify behavior of split method by empty match regex across all platforms

Deprecation cycle:

• <1.3: behavior of described calls is different when comparing JS, JRE 6, JRE 7 versus JRE 8+
• >=1.3: unify behavior across the platforms

Discontinued deprecated artifacts in the compiler distribution

Issue: KT-23799

Component: other

Incompatible change type: Binary

Short summary: Kotlin 1.3 discontinues the following deprecated binary artifacts:

• kotlin-runtime: use kotlin-stdlib instead
• kotlin-stdlib-jre7/8: use kotlin-stdlib-jdk7/8 instead
• kotlin-jslib in the compiler distribution: use kotlin-stdlib-js instead

Deprecation cycle:

• 1.2.X: the artifacts were marked as deprecated, the compiler reported warning on usage of those artifacts
• >=1.3: the artifacts are discontinued

Annotations in stdlib

Issue: KT-21784

Component: kotlin-stdlib (JVM)

Incompatible change type: Binary

Short summary: Kotlin 1.3 removes annotations from the package org.jetbrains.annotations from stdlib and moves them to the separate artifacts shipped with the compiler: annotations-13.0.jar and mutability-annotations-compat.jar

Deprecation cycle:

• <1.3: annotations were shipped with the stdlib artifact
• >=1.3: annotations ship in separate artifacts