[Gradle] GetCommonizerTargetOfSourceSet: Gracefully handle common non-hmpp workaround

libraries/tools/kotlin-gradle-plugin/src/functionalTest/kotlin/org/jetbrains/kotlin/gradle/SourceSetCommonizerTargetTest.kt

@@ -163,4 +163,18 @@ class SourceSetCommonizerTargetTest {
         assertNull(project.getCommonizerTarget(commonMain), "Expected commonMain to have no commonizer target")
         assertNull(project.getCommonizerTarget(commonTest), "Expected commonTest to have no commonizer target")
     }
+
+    @Test
+    fun `nativeMain with non hmpp workaround`() {
+        val linux1 = kotlin.linuxX64("linux1")
+        val linux2 = kotlin.linuxArm64("linux2")
+
+        val nativeMain = kotlin.sourceSets.create("nativeMain")
+
+        listOf(linux1, linux2).forEach { target ->
+            target.compilations.getByName("main").source(nativeMain)
+        }
+
+        assertNull(project.getCommonizerTarget(nativeMain), "Expected no commonizer target, since no real source set hierarchy is given")
+    }
 }

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/native/internal/GetCommonizerTargetOfSourceSet.kt

@@ -40,12 +40,13 @@ private fun Project.getLeafCommonizerTarget(sourceSet: KotlinSourceSet): LeafCom
     val konanTargets = compilationsBySourceSets(this)[sourceSet].orEmpty()
         .flatMap { compilation -> compilation.konanTargets }
 
-    return when {
-        konanTargets.isEmpty() -> null
-        konanTargets.size == 1 -> LeafCommonizerTarget(konanTargets.single())
-        else -> error("Source set ${sourceSet.name} is not a leaf source set. konanTargets=$konanTargets")
-    }
+    return if (konanTargets.size == 1) LeafCommonizerTarget(konanTargets.single())
 
+    /*
+    Can even be more than one, when added using `KotlinCompilation.source`. Still returning null, since this
+    does not represent a 'proper' source set hierarchy
+     */
+    else null
 }
 
 private fun KotlinSourceSetContainer.resolveSourceSetsDirectlyDependingOn(sourceSet: KotlinSourceSet): Set<KotlinSourceSet> {