Force compilation error with sealed classes

Question

With sealed classes you can use exhaustive when expressions and omit the else clause when the expression returns a result:

sealed c         


        

Answer 1

A discussion triggered me to look for a more general solution and found one, for Gradle builds. It doesn't require changing the source code! The drawback is that compilation may become noisy.

build.gradle.kts

tasks.withType().configureEach {
    val taskOutput = StringBuilder()
    logging.level = LogLevel.INFO
    logging.addStandardOutputListener { taskOutput.append(it) }
    doLast {
        fun CharSequence.hasInfoWithError(): Boolean =
            "'when' expression on sealed classes is recommended to be exhaustive" in this
        if (taskOutput.hasInfoWithError()) {
            throw Exception("kotlinc infos considered as errors found, see compiler output for details.")
        }
    }
}

build.gradle

tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).configureEach {
    def taskOutput = new StringBuilder()
    logging.level = LogLevel.INFO
    logging.addStandardOutputListener(new StandardOutputListener() {
        void onOutput(CharSequence text) { taskOutput.append(text) }
    })
    doLast {
        def hasInfoWithError = { CharSequence output ->
            output.contains("'when' expression on sealed classes is recommended to be exhaustive")
        }
        if (hasInfoWithError(taskOutput)) {
            throw new Exception("kotlinc infos considered as errors found, see compiler output for details.")
        }
    }
}

Notes:

• Change implementation of hasInfoWithError to generalize to other i:s.
• Put this code in subprojects { } or allprojects { } to apply project-wide.

References:

• Issue tracking making missing sealed case as warning:
  (which together with kotlinOptions.allWarningsAsErrors would solve the issue)
  https://youtrack.jetbrains.com/issue/KT-37651
• Discussion triggering this solution:
  https://youtrack.jetbrains.com/issue/KT-12380#focus=streamItem-27-4017839.0-0
• Repo with working example:
  https://github.com/TWiStErRob/repros/tree/master/kotlin/fail-on-sealed-when-info