Gradle plugin: don't register generated sources via generatedKotlin on KSP projects

wire-gradle-plugin/src/main/kotlin/com/squareup/wire/gradle/kotlin/SourceRoots.kt

@@ -141,12 +141,12 @@ private class JvmOrKmpSource(
           javaSourceDirectorySet?.srcDir(outputDirectory)
         }
         is KotlinOutput -> {
-          registerKotlinGeneratedSources(kotlinSourceSet, outputDirectory)
+          registerKotlinGeneratedSources(project, kotlinSourceSet, outputDirectory)
         }
         else -> {
           // Custom and third-party outputs are wildcards, so we add all output directories.
           javaSourceDirectorySet?.srcDir(outputDirectory)
-          registerKotlinGeneratedSources(kotlinSourceSet, outputDirectory)
+          registerKotlinGeneratedSources(project, kotlinSourceSet, outputDirectory)
         }
       }
     }
@@ -185,23 +185,34 @@ private class AndroidSource(
   }
 }
 
+private const val KSP_PLUGIN_ID = "com.google.devtools.ksp"
+
 /**
  * Registers [outputDirectory] as a generated Kotlin source directory on [kotlinSourceSet].
  *
  * On Kotlin 2.3+, uses the [KotlinSourceSet.generatedKotlin] API so that IDEs can distinguish
  * generated sources from handwritten ones. Falls back to [KotlinSourceSet.kotlin] on older
- * versions of the Kotlin Gradle Plugin where the API is not available.
+ * versions of the Kotlin Gradle Plugin where the API is not available, and on projects using
+ * KSP, which doesn't know about `generatedKotlin`: sources registered there are invisible to
+ * symbol processors, and KSP tasks lose the dependency on the Wire task they used to get from
+ * the `kotlin.srcDir()` registration, failing Gradle's execution-time dependency validation in
+ * builds which also reference Wire's output directory from another source set.
  */
 private fun registerKotlinGeneratedSources(
+  project: Project,
   kotlinSourceSet: KotlinSourceSet?,
   outputDirectory: Provider<Directory>,
 ) {
   if (kotlinSourceSet == null) return
   // generatedKotlin was introduced experimentally in Kotlin 2.3. Detect it reflectively so that
   // Wire remains compatible with earlier Kotlin Gradle Plugin versions.
-  val generatedKotlinMethod = runCatching {
-    kotlinSourceSet.javaClass.getMethod("getGeneratedKotlin")
-  }.getOrNull()
+  val generatedKotlinMethod = if (project.pluginManager.hasPlugin(KSP_PLUGIN_ID)) {
+    null
+  } else {
+    runCatching {
+      kotlinSourceSet.javaClass.getMethod("getGeneratedKotlin")
+    }.getOrNull()
+  }
   if (generatedKotlinMethod != null) {
     val generatedKotlin = generatedKotlinMethod.invoke(kotlinSourceSet) as SourceDirectorySet
     generatedKotlin.srcDir(outputDirectory)

wire-gradle-plugin/src/test/kotlin/com/squareup/wire/gradle/WirePluginTest.kt

@@ -713,6 +713,26 @@ class WirePluginTest {
     assertThat(result.task(":compileKotlin")).isNotNull()
   }
 
+  /**
+   * Verifies that projects using KSP build even though `KotlinSourceSet.generatedKotlin` is
+   * available. KSP doesn't know about `generatedKotlin`, so registering Wire's output there
+   * hides it from symbol processors and drops the task dependency KSP builds used to get from
+   * the `kotlin.srcDir()` registration, failing Gradle's dependency validation ("Task
+   * ':kspKotlin' uses this output of task ':generateMainProtos' without declaring an explicit
+   * or implicit dependency") when the output directory is also referenced from another source
+   * set. Wire falls back to `kotlin.srcDir()` on such projects.
+   */
+  @Test
+  fun kotlinProjectKotlinProtosWithGeneratedKotlinApiAndKsp() {
+    val fixtureRoot = File("src/test/projects/kotlin-project-kotlin-protos-kgp23-ksp")
+
+    val result = fixtureGradleRunner(fixtureRoot, "clean", "build").build()
+
+    assertThat(result.task(":generateMainProtos")).isNotNull()
+    assertThat(result.task(":kspKotlin")).isNotNull()
+    assertThat(result.task(":compileKotlin")).isNotNull()
+  }
+
   @Test
   fun protoLibrary() {
     val fixtureRoot = File("src/test/projects/proto-library")

wire-gradle-plugin/src/test/projects/kotlin-project-kotlin-protos-kgp23-ksp/build.gradle

@@ -0,0 +1,63 @@
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+
+buildscript {
+  dependencies {
+    classpath "com.squareup.wire:wire-gradle-plugin:$wireVersion"
+    // Hardcoded to a Kotlin version where the KotlinSourceSet.generatedKotlin API is available.
+    classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:2.3.10"
+    classpath "com.google.devtools.ksp:symbol-processing-gradle-plugin:2.3.10"
+  }
+
+  repositories {
+    maven {
+      url new File(rootDir, "../../../../../build/localMaven").toURI().toString()
+    }
+    mavenCentral()
+    google()
+  }
+}
+
+apply plugin: 'application'
+apply plugin: 'org.jetbrains.kotlin.jvm'
+apply plugin: 'com.google.devtools.ksp'
+apply plugin: 'com.squareup.wire'
+
+application.mainClass = "com.squareup.dinosaurs.Sample"
+
+repositories {
+  maven {
+    url new File(rootDir, "../../../../../build/localMaven").toURI().toString()
+  }
+  mavenCentral()
+}
+
+dependencies {
+  implementation "com.squareup.wire:wire-runtime:$wireVersion"
+  // Any processor will do: its presence creates the kspKotlin task, which consumes the
+  // main source set's Kotlin directories including Wire's generated sources.
+  ksp "com.squareup.moshi:moshi-kotlin-codegen:1.15.2"
+}
+
+// Mimics real-world builds which add Wire's output directory to a source set as a plain path,
+// with no task dependency. Wire's own registration of the same directory on the Kotlin source
+// set is what gives the task graph the edge from KSP to the Wire task.
+sourceSets {
+  main {
+    java.srcDir(layout.buildDirectory.dir("generated/source/wire"))
+  }
+}
+
+wire {
+  kotlin {}
+}
+
+tasks.withType(JavaCompile).configureEach {
+  sourceCompatibility = JavaVersion.VERSION_11.toString()
+  targetCompatibility = JavaVersion.VERSION_11.toString()
+}
+
+tasks.withType(KotlinCompile).configureEach {
+  kotlinOptions {
+    jvmTarget = "11"
+  }
+}

wire-gradle-plugin/src/test/projects/kotlin-project-kotlin-protos-kgp23-ksp/gradle.properties

@@ -0,0 +1,2 @@
+# The dependency-validation regression only manifests with KSP2's KspAATask.
+ksp.useKSP2=true

wire-gradle-plugin/src/test/projects/kotlin-project-kotlin-protos-kgp23-ksp/settings.gradle

@@ -0,0 +1 @@
+include ':'

wire-gradle-plugin/src/test/projects/kotlin-project-kotlin-protos-kgp23-ksp/src/main/java/com/squareup/dinosaurs/Sample.kt

@@ -0,0 +1,33 @@
+/*
+ * Copyright (C) 2026 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.squareup.dinosaurs
+
+import com.squareup.geology.Period
+import java.io.IOException
+import okio.ByteString.Companion.toByteString
+
+/* Note that we do not execute this class in tests. We're only testing that it compiles. */
+fun main(args: Array<String>) {
+  val stegosaurus = Dinosaur(
+    name = "Stegosaurus",
+    period = Period.JURASSIC,
+    length_meters = 9.0,
+    mass_kilograms = 5_000.0,
+    picture_urls = listOf("http://goo.gl/LD5KY5", "http://goo.gl/VYRM67"),
+  )
+  val stegosaurusEncoded = Dinosaur.ADAPTER.encode(stegosaurus)
+  println(stegosaurusEncoded.toByteString().base64())
+}

wire-gradle-plugin/src/test/projects/kotlin-project-kotlin-protos-kgp23-ksp/src/main/proto/squareup/dinosaurs/dinosaur.proto

@@ -0,0 +1,19 @@
+syntax = "proto2";
+
+package squareup.dinosaurs;
+
+option java_package = "com.squareup.dinosaurs";
+
+import "squareup/geology/period.proto";
+
+message Dinosaur {
+  /** Common name of this dinosaur, like "Stegosaurus". */
+  optional string name = 1;
+
+  /** URLs with images of this dinosaur. */
+  repeated string picture_urls = 2;
+
+  optional double length_meters = 3;
+  optional double mass_kilograms = 4;
+  optional squareup.geology.Period period = 5;
+}

wire-gradle-plugin/src/test/projects/kotlin-project-kotlin-protos-kgp23-ksp/src/main/proto/squareup/geology/period.proto

@@ -0,0 +1,16 @@
+syntax = "proto2";
+
+package squareup.geology;
+
+option java_package = "com.squareup.geology";
+
+enum Period {
+  /** 145.5 million years ago — 66.0 million years ago. */
+  CRETACEOUS = 1;
+
+  /** 201.3 million years ago — 145.0 million years ago. */
+  JURASSIC = 2;
+
+  /** 252.17 million years ago — 201.3 million years ago. */
+  TRIASSIC = 3;
+}