Motivation types:

• Problematic/meaningless usage patterns need to be discouraged/blocked (e.g. counterintuitive behaviors)
  • Code is error-prone
• Inconsistency in the design (things are done differently in different contexts)

Impact types:

• Compilation. Some code that used to compile won't compile any more
  • There were cases when such code worked with no exceptions
    • Some such code could compile without any warnings

More details: KT-47709: Make when statements with enum, sealed, and Boolean subjects exhaustive by default

The quick-fix adds the missing else -> {} branch.

Example:

  sealed class Base {
      class A : Base()
      class B : Base()
  }

  fun test(base: Base) {
      when (base) {
          is Base.A -> ""
      }
  }

After the quick-fix is applied:

  sealed class Base {
      class A : Base()
      class B : Base()
  }

  fun test(base: Base) {
      when (base) {
          is Base.A -> ""
          else -> {}
      }
  }

This inspection only reports if the Kotlin language level of the project or module is 1.6 or higher.