Kotlin Multiplatform "expect/actual" mismatch in commonMain in CI

In Kotlin Multiplatform, expect declarations in commonMain require a matching actual in every compiled target sourceSet. CI fails when a target compiled on the runner has no actual, or when signatures drift, because the mismatch is only checked when that target is built.

What this error means

A target compile task fails with "expected declaration has no actual declaration in module for JVM" or "actual ... has no corresponding expected declaration".

> Task :shared:compileKotlinJvm FAILED
e: /shared/src/commonMain/kotlin/Platform.kt: (3, 1): Expected function 'currentTime' has no
actual declaration in module shared.jvmMain

Common causes

A target sourceSet is missing an actual

CI builds a target (jvm, js, a native one) whose sourceSet never provided the actual for a common expect, so that target fails to compile.

Signatures drifted between expect and actual

A parameter or return type changed on one side, so the actual no longer matches the expect for that target.

How to fix it

Provide an actual for every compiled target

1. Read which module/target the error names.
2. Add the matching actual in that target sourceSet with an identical signature.
3. Rebuild that target to confirm the pair matches.

// commonMain
expect fun currentTime(): Long
// jvmMain
actual fun currentTime(): Long = System.currentTimeMillis()

Keep expect/actual signatures identical

Ensure names, parameters, and return types match exactly across all target sourceSets you compile in CI.

How to prevent it

• Add the actual in every target sourceSet the CI matrix builds.
• Keep expect and actual signatures byte-for-byte aligned.
• Compile all declared targets in CI so a missing actual is caught early.

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