Fallback to publishing with coordinates inferred from publication

In Gradle < 4.7, there's no `ComponentWithCoordinates` interface, and we
need to make sure that none of our classes implementing this interface
are loaded with older Gradle versions.

Instead of that interface, we can rely on Gradle's metadata generation
heuristic that takes Maven coordinates for the variants from the
corresponding publications.

libraries/tools/kotlin-gradle-plugin-api/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinTarget.kt

@@ -9,10 +9,10 @@ import org.gradle.api.Named
 import org.gradle.api.NamedDomainObjectContainer
 import org.gradle.api.Project
 import org.gradle.api.attributes.HasAttributes
-import org.gradle.api.component.ComponentWithCoordinates
+import org.gradle.api.component.SoftwareComponent
 import org.gradle.api.internal.component.UsageContext
 
-interface KotlinTargetComponent : ComponentWithCoordinates {
+interface KotlinTargetComponent : SoftwareComponent {
     val target: KotlinTarget
     val publishable: Boolean
 }

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/mpp/KotlinMultiplatformPlugin.kt

@@ -106,15 +106,15 @@ class KotlinMultiplatformPlugin(
 
                 // Create separate publications for all publishable targets
                 kotlinSoftwareComponent.variants.filter { it.publishable }.forEach { variant ->
-                    val coordinates = variant.coordinates
+                    val coordinates = (variant as? KotlinVariantWithCoordinates)?.coordinates
                     val name = variant.target.disambiguateName("kotlin")
 
                     publishing.publications.create(name, MavenPublication::class.java) { publication ->
                         publication.from(variant)
                         (publication as MavenPublicationInternal).publishWithOriginalFileName()
-                        publication.artifactId = coordinates.name
+                        publication.artifactId = coordinates?.name ?: "${project.name}-${variant.target.name.toLowerCase()}"
-                        publication.groupId = coordinates.group
+                        publication.groupId = coordinates?.group ?: project.group.toString()
-                        publication.version = coordinates.version
+                        publication.version = coordinates?.version ?: project.version.toString()
                     }
                 }
             }

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/mpp/KotlinSoftwareComponent.kt

@@ -10,8 +10,8 @@ import org.gradle.api.artifacts.*
 import org.gradle.api.attributes.AttributeContainer
 import org.gradle.api.attributes.Usage
 import org.gradle.api.capabilities.Capability
+import org.gradle.api.component.ComponentWithCoordinates
 import org.gradle.api.component.ComponentWithVariants
-import org.gradle.api.internal.artifacts.DefaultModuleVersionIdentifier
 import org.gradle.api.internal.component.SoftwareComponentInternal
 import org.gradle.api.internal.component.UsageContext
 import org.jetbrains.kotlin.gradle.plugin.KotlinTarget
@@ -37,17 +37,9 @@ class KotlinSoftwareComponent(
     }
 }
 
-class KotlinVariant(
+open class KotlinVariant(
-    private val project: Project,
     override val target: KotlinTarget
 ) : SoftwareComponentInternal, KotlinTargetComponent {
-
-    override fun getCoordinates() = DefaultModuleVersionIdentifier(
-        project.group.toString(),
-        "${project.name}-${target.name.toLowerCase()}",
-        project.version.toString()
-    )
-
     override fun getUsages(): Set<UsageContext> = target.createUsageContexts()
     override fun getName(): String = target.name
 
@@ -55,6 +47,28 @@ class KotlinVariant(
         get() = target.publishable
 }
 
+class KotlinVariantWithCoordinates(
+    target: KotlinTarget
+) : KotlinVariant(target),
+    ComponentWithCoordinates /* Gradle 4.7+ API, don't use with older versions */
+{
+    override fun getCoordinates() = object : ModuleVersionIdentifier {
+        private val project get() = target.project
+
+        private val moduleName: String get() = "${project.name}-${target.name.toLowerCase()}"
+        private val moduleGroup: String get() = project.group.toString()
+
+        override fun getGroup() = moduleGroup
+        override fun getName() = moduleName
+        override fun getVersion() = project.version.toString()
+
+        override fun getModule(): ModuleIdentifier = object : ModuleIdentifier {
+            override fun getGroup(): String = moduleGroup
+            override fun getName(): String = moduleName
+        }
+    }
+}
+
 // At the moment all KN artifacts have JAVA_API usage.
 // TODO: Replace it with a specific usage
 object NativeUsage {

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/mpp/kotlinTargets.kt

@@ -12,6 +12,7 @@ import org.gradle.api.attributes.AttributeContainer
 import org.gradle.api.internal.component.UsageContext
 import org.gradle.api.plugins.JavaPlugin
 import org.jetbrains.kotlin.gradle.plugin.*
+import org.jetbrains.kotlin.gradle.utils.isGradleVersionAtLeast
 import org.jetbrains.kotlin.gradle.utils.lowerCamelCaseName
 import org.jetbrains.kotlin.konan.target.KonanTarget
 
@@ -39,7 +40,12 @@ abstract class AbstractKotlinTarget (
     override val publishable: Boolean
         get() = true
 
-    override val component: KotlinTargetComponent = KotlinVariant(project, this)
+    override val component: KotlinTargetComponent by lazy {
+        if (isGradleVersionAtLeast(4, 7))
+            KotlinVariantWithCoordinates(this)
+        else
+            KotlinVariant(this)
+    }
 
     override fun createUsageContexts(): Set<UsageContext> =
         setOf(