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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.

Usually incompatible changes happen only in feature releases, but this time we have to introduce two such changes in an incremental release to limit spread of the problems introduced by changes in Kotlin 1.7.

This document summarizes them, providing a reference for migration from Kotlin 1.7.0 and 1.7.10 to Kotlin 1.7.20.

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

Rollback attempt to fix proper constraints processing

Issue: KT-53813

Component: Core language

Incompatible change type: source

Short summary: Rollback an attempt of fixing issues in type inference constraints processing appeared in 1.7.0 after implementing the change described in KT-52668. The attempt was made in 1.7.10, but it in turn introduced new problems.

Deprecation cycle:

  • 1.7.20: Rollback to 1.7.0 behavior

Forbid some builder inference cases to avoid problematic interaction with multiple lambdas and resolution

Issue: KT-53797

Component: Core language

Incompatible change type: source

Short summary: Kotlin 1.7 introduced a feature called unrestricted builder inference, so that even the lambdas passed to parameters not annotated with @BuilderInference could benefit from the builder inference. However, that could cause several problems if more than one such lambda occurred in a function invocation.

Kotlin 1.7.20 will report an error if more than one lambda function having the corresponding parameter not annotated with @BuilderInference requires using builder inference to complete inferring the types in the lambda.

Deprecation cycle:

  • 1.7.20: report an error on such lambda functions,
    -XXLanguage:+NoBuilderInferenceWithoutAnnotationRestriction can be used to temporarily revert to the pre-1.7.20 behavior