Fix internals of an android-library not accessible in its androidTest

Issue #KT-19370 Fixed

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/AndroidProject/Lib/src/androidTest/kotlin/internalTest.kt

@@ -0,0 +1,5 @@
+import lib.internalLibFun
+
+fun main(args: Array<String>) {
+    internalLibFun()
+}

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/AndroidProject/Lib/src/main/kotlin/lib/libUtil.kt

@@ -1,3 +1,5 @@
 package lib
 
 fun libUtil(): String = "libUtil"
+
+internal fun internalLibFun() = "internalLibFun"

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

@@ -4,6 +4,7 @@ import com.android.build.gradle.BaseExtension
 import com.android.build.gradle.BasePlugin
 import com.android.build.gradle.internal.variant.BaseVariantData
 import com.android.build.gradle.internal.variant.BaseVariantOutputData
+import com.android.build.gradle.internal.variant.LibraryVariantData
 import com.android.build.gradle.internal.variant.TestVariantData
 import com.android.builder.model.SourceProvider
 import org.gradle.api.Project
@@ -14,6 +15,7 @@ import org.jetbrains.kotlin.gradle.internal.KaptVariantData
 import org.jetbrains.kotlin.gradle.internal.registerGeneratedJavaSource
 import org.jetbrains.kotlin.gradle.plugin.android.AndroidGradleWrapper
 import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+import org.jetbrains.kotlin.gradle.utils.checkedReflection
 import org.jetbrains.kotlin.incremental.configureMultiProjectIncrementalCompilation
 import org.jetbrains.kotlin.incremental.multiproject.ArtifactDifferenceRegistryProviderAndroidWrapper
 import java.io.File
@@ -57,8 +59,29 @@ internal class LegacyAndroidAndroidProjectHandler(kotlinConfigurationTools: Kotl
 
         configureJavaTask(kotlinTask, javaTask, logger)
         createSyncOutputTask(project, kotlinTask, javaTask, kotlinAfterJavaTask, getVariantName(variantData))
+
+        // In lib modules, the androidTest variants get the classes jar in their classpath instead of the Java
+        // destination dir. To use it as a friend path, set the jar as the javaOutputDir (see its usages):
+        if (variantData is LibraryVariantData) {
+            variantData.dependencyJarOrNull?.let { kotlinTask.javaOutputDir = it }
+        }
     }
 
+    private val LibraryVariantData.dependencyJarOrNull: File?
+        get() =
+            checkedReflection(f@{
+                val output = variantConfiguration.javaClass.methods.firstOrNull { it.name == "getOutput" }
+                        ?.invoke(variantConfiguration)
+                        ?: return@f null
+
+                output.javaClass.methods?.firstOrNull { it.name == "getJarFile" }
+                        ?.invoke(output) as? File
+
+            }, { e: Exception ->
+                logger.kotlinDebug("dependencyJarOrNull for lib variant $name failed due to $e")
+                null
+            })
+
     override fun getVariantName(variant: BaseVariantData<out BaseVariantOutputData>): String = variant.name
 
     override fun checkVariantIsValid(variant: BaseVariantData<out BaseVariantOutputData>): Unit {

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/utils/reflectionUtils.kt

@@ -17,6 +17,7 @@
 package org.jetbrains.kotlin.gradle.utils
 
 import java.lang.reflect.Field
+import java.lang.reflect.InvocationTargetException
 
 internal fun Class<*>.getDeclaredFieldInHierarchy(name: String): Field? {
     val inheritanceChain = generateSequence(this) { it.superclass }
@@ -28,3 +29,18 @@ internal fun Class<*>.getDeclaredFieldInHierarchy(name: String): Field? {
         }
     }.filterNotNull().first()
 }
+
+internal inline fun <T> checkedReflection(block: () -> T, onReflectionException: (Exception) -> T): T {
+    return try {
+        block()
+    }
+    catch (e: InvocationTargetException) {
+        throw e.targetException
+    }
+    catch (e: ReflectiveOperationException) {
+        onReflectionException(e)
+    }
+    catch (e: IllegalArgumentException) {
+        onReflectionException(e)
+    }
+}