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Compatibility Guide for Kotlin 1.4

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 1.3 to Kotlin 1.4.

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.

Unexpected behavior with in infix operator and ConcurrentHashMap

Issue: KT-18053

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.4 will outlaw auto operator contains coming from the implementors of java.util.Map written in Java

Deprecation cycle:

  • < 1.4: introduce warning for problematic operators at call-site
  • >= 1.4: raise this warning to an error, -XXLanguage:-ProhibitConcurrentHashMapContains can be used to temporarily revert to pre-1.4 behavior

Prohibit access to protected members inside public inline members

Issue: KT-21178

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.4 will prohibit access to protected members from public inline members.

Deprecation cycle:

  • < 1.4: introduce warning at call-site for problematic cases
  • 1.4: raise this warning to an error, -XXLanguage:-ProhibitProtectedCallFromInline can be used to temporarily revert to pre-1.4 behavior

Contracts on calls with implicit receivers

Issue: KT-28672

Component: Core Language

Incompatible change type: behavioral

Short summary: smart casts from contracts will be available on calls with implicit receivers in 1.4

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-ContractsOnCallsWithImplicitReceiver can be used to temporarily revert to pre-1.4 behavior

Inconsistent behavior of floating-point number comparisons

Issues: KT-22723

Component: Core language

Incompatible change type: behavioral

Short summary: since Kotlin 1.4, Kotlin compiler will use IEEE 754 standard to compare floating-point numbers

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-ProperIeee754Comparisons can be used to temporarily revert to pre-1.4 behavior

No smart cast on the last expression in a generic lambda

Issue: KT-15020

Component: Core Language

Incompatible change type: behavioral

Short summary: smart casts for last expressions in lambdas will be correctly applied since 1.4

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-NewInference can be used to temporarily revert to pre-1.4 behavior. Note that this flag will also disable several new language features.

Do not depend on the order of lambda arguments to coerce result to Unit

Issue: KT-36045

Component: Core language

Incompatible change type: source

Short summary: since Kotlin 1.4, lambda arguments will be resolved independently without implicit coercion to Unit

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-NewInference can be used to temporarily revert to pre-1.4 behavior. Note that this flag will also disable several new language features.

Wrong common supertype between raw and integer literal type leads to unsound code

Issue: KT-35681

Components: Core language

Incompatible change type: source

Short summary: since Kotlin 1.4, common supertype between raw Comparable type and integer literal type will be more specific

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-NewInference can be used to temporarily revert to pre-1.4 behavior. Note that this flag will also disable several new language features.

Type safety problem because several equal type variables are instantiated with a different types

Issue: KT-35679

Component: Core language

Incompatible change type: source

Short summary: since Kotlin 1.4, Kotlin compiler will prohibit instantiating equal type variables with different types

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-NewInference can be used to temporarily revert to pre-1.4 behavior. Note that this flag will also disable several new language features.

Type safety problem because of incorrect subtyping for intersection types

Issues: KT-22474

Component: Core language

Incompatible change type: source

Short summary: in Kotlin 1.4, subtyping for intersection types will be refined to work more correctly

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-NewInference can be used to temporarily revert to pre-1.4 behavior. Note that this flag will also disable several new language features.

No type mismatch with an empty when expression inside lambda

Issue: KT-17995

Component: Core language

Incompatible change type: source

Short summary: since Kotlin 1.4, there will be a type mismatch for empty when expression if it's used as the last expression in a lambda

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-NewInference can be used to temporarily revert to pre-1.4 behavior. Note that this flag will also disable several new language features.

Return type Any inferred for lambda with early return with integer literal in one of possible return values

Issue: KT-20226

Component: Core language

Incompatible change type: source

Short summary: since Kotlin 1.4, integer type returning from a lambda will be more specific for cases when there is early return

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-NewInference can be used to temporarily revert to pre-1.4 behavior. Note that this flag will also disable several new language features.

Proper capturing of star projections with recursive types

Issue: KT-33012

Component: Core language

Incompatible change type: source

Short summary: since Kotlin 1.4, more candidates will become applicable because capturing for recursive types will work more correctly

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-NewInference can be used to temporarily revert to pre-1.4 behavior. Note that this flag will also disable several new language features.

Common supertype calculation with non-proper type and flexible one leads to incorrect results

Issue: KT-37054

Component: Core language

Incompatible change type: behavioral

Short summary: since Kotlin 1.4, common supertype between flexible types will be more specific protecting from runtime errors

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-NewInference can be used to temporarily revert to pre-1.4 behavior. Note that this flag will also disable several new language features.

Type safety problem because of lack of captured conversion against nullable type argument

Issue: KT-35487

Component: Core language

Incompatible change type: source

Short summary: since Kotlin 1.4, subtyping between captured and nullable types will be more correct protecting from runtime errors

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-NewInference can be used to temporarily revert to pre-1.4 behavior. Note that this flag will also disable several new language features.

Preserve intersection type for covariant types after unchecked cast

Issue: KT-37280

Component: Core language

Incompatible change type: source

Short summary: since Kotlin 1.4, uchecked casts of covariant types produce the intersection type for smart casts, not the type of the unchecked cast.

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-NewInference can be used to temporarily revert to pre-1.4 behavior. Note that this flag will also disable several new language features.

Type variable leaks from builder inference because of using this expression

Issue: KT-32126

Component: Core language

Incompatible change type: source

Short summary: since Kotlin 1.4, using this inside builder functions like sequence {} is prohibited if there are no other proper constraints

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-NewInference can be used to temporarily revert to pre-1.4 behavior. Note that this flag will also disable several new language features.

Wrong overload resolution for contravariant types with nullable type arguments

Issue: KT-31670

Component: Core language

Incompatible change type: source

Short summary: since Kotlin 1.4, if two overloads of a function that takes contravariant type arguments differ only by the nullability of the type (such as In<T> and In<T?>), the nullable type is considered more specific.

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-NewInference can be used to temporarily revert to pre-1.4 behavior. Note that this flag will also disable several new language features.

Builder inference with non-nested recursive constraints

Issue: KT-34975

Component: Core language

Incompatible change type: source

Short summary: since Kotlin 1.4, builder functions such as sequence {} with type that depends on a recursive constraint inside the passed lambda cause a compiler error.

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-NewInference can be used to temporarily revert to pre-1.4 behavior. Note that this flag will also disable several new language features.

Eager type variable fixation leads to a contradictory constraint system

Issue: KT-25175

Component: Core language

Incompatible change type: source

Short summary: since Kotlin 1.4, the type inference in certain cases works less eagerly allowing to find the constraint system that is not contradictory.

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-NewInference can be used to temporarily revert to pre-1.4 behavior. Note that this flag will also disable several new language features.

Prohibit tailrec modifier onopen functions

Issue: KT-18541

Component: Core language

Incompatible change type: source

Short summary: since Kotlin 1.4, functions can't have open and tailrec modifiers at the same time.

Deprecation cycle:

  • < 1.4: report a warning on functions that have open and tailrec modifiers together (error in the progressive mode).
  • >= 1.4: raise this warning to an error.

The INSTANCE field of a companion object more visible than the companion object class itself

Issue: KT-11567

Component: Kotlin/JVM

Incompatible change type: source

Short summary: since Kotlin 1.4, if a companion object is private, then its field INSTANCE will be also private

Deprecation cycle:

  • < 1.4: the compiler generates object INSTANCE with a deprecated flag
  • >= 1.4: companion object INSTANCE field has proper visibility

Outer finally block inserted before return is not excluded from thecatch interval of the inner try block without finally

Issue: KT-31923

Component: Kotlin/JVM

Incompatible change type: behavioral

Short summary: since Kotlin 1.4, the catch interval will be computed properly for nested try/catch blocks

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-ProperFinally can be used to temporarily revert to pre-1.4 behavior

Use the boxed version of an inline class in return type position for covariant and generic-specialized overrides

Issues: KT-30419

Component: Kotlin/JVM

Incompatible change type: behavioral

Short summary: since Kotlin 1.4, functions using covariant and generic-specialized overrides will return boxed values of inline classes

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed

Do not declare checked exceptions in JVM bytecode when using delegation to Kotlin interfaces

Issue: KT-35834

Component: Kotlin/JVM

Incompatible change type: source

Short summary: Kotlin 1.4 will not generate checked exceptions during interface delegation to Kotlin interfaces

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-DoNotGenerateThrowsForDelegatedKotlinMembers can be used to temporarily revert to pre-1.4 behavior

Changed behavior of signature-polymorphic calls to methods with a single vararg parameter to avoid wrapping the argument into another array

Issue: KT-35469

Component: Kotlin/JVM

Incompatible change type: source

Short summary: Kotlin 1.4 will not wrap the argument into another array on a signature-polymorphic call

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed

Incorrect generic signature in annotations when KClass is used as a generic parameter

Issue: KT-35207

Component: Kotlin/JVM

Incompatible change type: source

Short summary: Kotlin 1.4 will fix incorrect type mapping in annotations when KClass is used as a generic parameter

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed

Forbid spread operator in signature-polymorphic calls

Issue: KT-35226

Component: Kotlin/JVM

Incompatible change type: source

Short summary: Kotlin 1.4 will prohibit the use of spread operator (*) on signature-polymorphic calls

Deprecation cycle:

  • < 1.4: report a warning on the use of a spread operator in signature-polymorphic calls
  • >= 1.5: raise this warning to an error, -XXLanguage:-ProhibitSpreadOnSignaturePolymorphicCall can be used to temporarily revert to pre-1.4 behavior

Change initialization order of default values for tail-recursive optimized functions

Issue: KT-31540

Component: Kotlin/JVM

Incompatible change type: behavioral

Short summary: Since Kotlin 1.4, the initialization order for tail-recursive functions will be the same as for regular functions

Deprecation cycle:

  • < 1.4: report a warning at declaration-site for problematic functions
  • >= 1.4: behavior changed, -XXLanguage:-ProperComputationOrderOfTailrecDefaultParameters can be used to temporarily revert to pre-1.4 behavior

Do not generate ConstantValue attribute for non-const vals

Issue: KT-16615

Component: Kotlin/JVM

Incompatible change type: behavioral

Short summary: Since Kotlin 1.4, the compiler will not generate the ConstantValue attribute for vals annotated with @JvmField

Deprecation cycle:

  • < 1.4: report a warning through an IntelliJ IDEA inspection
  • >= 1.4: behavior changed, -XXLanguage:-NoConstantValueAttributeForNonConstVals can be used to temporarily revert to pre-1.4 behavior

Generated overloads for @JvmOverloads on open methods should be final

Issue: KT-33240

Components: Kotlin/JVM

Incompatible change type: source

Short summary: overloads for functions with @JvmOverloads will be generated as final

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed, -XXLanguage:-GenerateJvmOverloadsAsFinal can be used to temporarily revert to pre-1.4 behavior

Lambdas returning kotlin.Result now return boxed value instead of unboxed

Issue: KT-39198

Component: Kotlin/JVM

Incompatible change type: behavioral

Short summary: since Kotlin 1.4, lambdas returning values of kotlin.Result type will return boxed value instead of unboxed

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed

Unify exceptions from null checks

Issue: KT-22275

Component: Kotlin/JVM

Incompatible change type: behavior

Short summary: Starting from Kotlin 1.4, all runtime null checks will throw a java.lang.NullPointerException

Deprecation cycle:

  • < 1.4: runtime null checks throw different exceptions, such as KotlinNullPointerException, IllegalStateException, IllegalArgumentException, and TypeCastException
  • >= 1.4: all runtime null checks throw a java.lang.NullPointerException

Comparing floating point values in array/list operations contains, indexOf, lastIndexOf: IEEE 754 or total order

Issue: KT-28753

Component: kotlin-stdlib (JVM)

Incompatible change type: behavioral

Short summary: the List implementation returned from Double/FloatArray.asList() will implement contains, indexOf, and lastIndexOf, so that they use total order equality

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed

Gradually change the return type of collection min and max functions to non-nullable

Issue: KT-38854

Component: kotlin-stdlib (JVM)

Incompatible change type: source

Short summary: return type of collection min and max functions will be changed to non-nullable in 1.6

Deprecation cycle:

  • 1.4: introduce ...OrNull functions as synonyms and deprecate the affected API (see details in the issue)
  • 1.5.x: raise the deprecation level of the affected API to error
  • >=1.6: reintroduce the affected API but with non-nullable return type

Deprecate appendln in favor of appendLine

Issue: KT-38754

Component: kotlin-stdlib (JVM)

Incompatible change type: source

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

Deprecation cycle:

  • 1.4: introduce appendLine function as a replacement for appendln and deprecate appendln
  • >=1.5: raise the deprecation level to error

Deprecate conversions of floating-point types to Short and Byte

Issue: KT-30360

Component: kotlin-stdlib (JVM)

Incompatible change type: source

Short summary: since Kotlin 1.4, conversions of floating-point types to Short and Byte will be deprecated

Deprecation cycle:

  • 1.4: deprecate Double.toShort()/toByte() and Float.toShort()/toByte() and propose replacement
  • >=1.5: raise the deprecation level to error

Fail fast in Regex.findAll on an invalid startIndex

Issue: KT-28356

Component: kotlin-stdlib

Incompatible change type: behavioral

Short summary: since Kotlin 1.4, findAll will be improved to check that startIndex is in the range of the valid position indices of the input char sequence at the moment of entering findAll, and throw IndexOutOfBoundsException if it's not

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed

Remove deprecated kotlin.coroutines.experimental

Issue: KT-36083

Component: kotlin-stdlib

Incompatible change type: source

Short summary: since Kotlin 1.4, the deprecated kotlin.coroutines.experimental API is removed from stdlib

Deprecation cycle:

  • < 1.4: kotlin.coroutines.experimental is deprecated with the ERROR level
  • >= 1.4: kotlin.coroutines.experimental is removed from stdlib. On the JVM, a separate compatibility artifact is provided (see details in the issue).

Remove deprecated mod operator

Issue: KT-26654

Component: kotlin-stdlib

Incompatible change type: source

Short summary: since Kotlin 1.4, mod operator on numeric types is removed from stdlib

Deprecation cycle:

  • < 1.4: mod is deprecated with the ERROR level
  • >= 1.4: mod is removed from stdlib

Hide Throwable.addSuppressed member and prefer extension instead

Issue: KT-38777

Component: kotlin-stdlib

Incompatible change type: behavior

Short summary: Throwable.addSuppressed() extension function is now preferred over the Throwable.addSuppressed() member function

Deprecation cycle:

  • < 1.4: old behavior (see details in the issue)
  • >= 1.4: behavior changed

capitalize should convert digraphs to title case

Issue: KT-38817

Component: kotlin-stdlib

Incompatible change type: behavior

Short summary: String.capitalize() function now capitalizes digraphs from the Serbo-Croatian Gaj's Latin alphabet in the title case (Dž instead of DŽ)

Deprecation cycle:

  • < 1.4: digraphs are capitalized in the upper case (DŽ)
  • >= 1.4: digraphs are capitalized in the title case (Dž)