Add a check of the AGP version

gradle-plugin/src/main/kotlin/proguard/gradle/plugin/ProGuardPlugin.kt

@@ -25,6 +25,7 @@ import com.android.build.gradle.BaseExtension
 import org.gradle.api.GradleException
 import org.gradle.api.Plugin
 import org.gradle.api.Project
+import org.gradle.util.VersionNumber
 import proguard.gradle.plugin.android.AndroidPlugin
 
 class ProGuardPlugin : Plugin<Project> {
@@ -41,6 +42,13 @@ class ProGuardPlugin : Plugin<Project> {
                                 |     }""".trimMargin()
         when {
             androidExtension != null -> {
+                if (!isAGPVersionSupported(project)) {
+                    throw GradleException(
+                            """The ProGuard plugin only supports Android plugin 4 and higher.
+                              |For Android plugin version 3 and lower, you can use ProGuard through the Android plugin integration.
+                              |Please refer to the manual for further details: https://www.guardsquare.com/manual/setup/gradleplugin
+                            """.trimMargin())
+                }
                 AndroidPlugin(androidExtension).apply(project)
             }
             javaExtension != null -> {
@@ -57,4 +65,13 @@ class ProGuardPlugin : Plugin<Project> {
             }
         }
     }
+
+    private fun isAGPVersionSupported(project: Project): Boolean {
+        val version = project.rootProject.buildscript.configurations.getByName("classpath").resolvedConfiguration.resolvedArtifacts.find { module ->
+            module.moduleVersion.id.group.equals("com.android.tools.build")
+        }?.moduleVersion?.id?.version
+        return version?.let {
+            return VersionNumber.parse(it).major >= 4
+        } ?: false
+    }
 }

gradle-plugin/src/test/kotlin/proguard/gradle/ProGuardPluginTest.kt

@@ -34,4 +34,58 @@ class ProGuardPluginTest : FreeSpec({
             }
         }
     }
+
+    "Given a project with an old Android Gradle plugin" - {
+        val project = autoClose(AndroidProject("""
+            buildscript {
+                repositories {
+                    mavenCentral() // For anything else.
+                    google()       // For the Android plugin.
+                    flatDir {
+                        dirs "${System.getProperty("local.repo")}"
+                    }
+                }
+                dependencies {
+                    classpath "com.android.tools.build:gradle:3.6.3"
+                    classpath ":proguard-gradle:${System.getProperty("proguard.version")}"
+                }
+            }
+            allprojects {
+                repositories {
+                    google()
+                    mavenCentral()
+                }
+            }
+            """.trimIndent()).apply {
+                addModule(applicationModule("app", buildDotGradle = """
+                            plugins {
+                                id 'com.android.application'
+                                id 'proguard'
+                            }
+
+                            android {
+                                compileSdkVersion 30
+
+                                buildTypes {
+                                    release {}
+                                    debug   {}
+                                }
+                            }
+
+                            proguard {
+                                configurations {
+                                    release {}
+                                }
+                            }
+                            """.trimIndent()))
+            }.create())
+
+        "When the project is evaluated" - {
+            val result = createGradleRunner(project.rootDir, testKitDir).buildAndFail()
+
+            "Then the build should fail" {
+                result.output shouldContain "The ProGuard plugin only supports Android plugin 4 and higher."
+            }
+        }
+    }
 })