Type parameter can have 'in' or 'out' variance

Reports type parameters that can have in or out variance.

Using in and out variance provides more precise type inference in Kotlin and clearer code semantics.

Example:

  class Box<T>(val obj: T)

  fun consumeString(box: Box<String>) {}
  fun consumeCharSequence(box: Box<CharSequence>) {}

  fun usage(box: Box<String>) {
      consumeString(box)
      consumeCharSequence(box) // Compilation error
  }

The quick-fix adds the matching variance modifier:

  class Box<out T>(val obj: T)

  fun consumeString(box: Box<String>) {}
  fun consumeCharSequence(box: Box<CharSequence>) {}

  fun usage(box: Box<String>) ++{
      consumeString(box)
      consumeCharSequence(box) // OK
  }

Locating this inspection

By ID

Can be used to locate inspection in e.g. Qodana configuration files, where you can quickly enable or disable it, or adjust its settings.

AddVarianceModifier

Via Settings dialog

Path to the inspection settings via IntelliJ Platform IDE Settings dialog, when you need to adjust inspection settings directly from your IDE.

Settings or Preferences | Editor | Inspections | Kotlin | Style issues

Inspection Details
By default bundled with:	IntelliJ IDEA 2024.2, Qodana for JVM 2024.2,
Can be installed with plugin:	Kotlin, 242.22892-IJ

Last modified: 11 September 2024