Setup the test sourceset better when using split sources.

src/main/java/net/fabricmc/loom/configuration/providers/minecraft/MinecraftSourceSets.java

@@ -205,6 +205,12 @@ private void evaluate(Project project) {
 			extendsFrom(project, clientOnlySourceSet.getCompileClasspathConfigurationName(), mainSourceSet.getCompileClasspathConfigurationName());
 			extendsFrom(project, clientOnlySourceSet.getRuntimeClasspathConfigurationName(), mainSourceSet.getRuntimeClasspathConfigurationName());
 
+			// Test source set depends on client
+			final SourceSet testSourceSet = SourceSetHelper.getSourceSetByName(SourceSet.TEST_SOURCE_SET_NAME, project);
+			extendsFrom(project, testSourceSet.getCompileClasspathConfigurationName(), clientOnlySourceSet.getCompileClasspathConfigurationName());
+			extendsFrom(project, testSourceSet.getRuntimeClasspathConfigurationName(), clientOnlySourceSet.getRuntimeClasspathConfigurationName());
+			project.getDependencies().add(testSourceSet.getImplementationConfigurationName(), clientOnlySourceSet.getOutput());
+
 			RemapConfigurations.configureClientConfigurations(project, clientOnlySourceSet);
 
 			// Include the client only output in the jars

src/test/groovy/net/fabricmc/loom/test/integration/SplitProjectTest.groovy

@@ -43,6 +43,7 @@ class SplitProjectTest extends Specification implements GradleProjectTestTrait {
 
 		then:
 		result.task(":build").outcome == SUCCESS
+		result.task(":test").outcome == SUCCESS
 
 		where:
 		version << STANDARD_TEST_VERSIONS

src/test/groovy/net/fabricmc/loom/test/util/ExtractTestProject.groovy

@@ -0,0 +1,83 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2024 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.test.util
+
+// A helper script to extract the test project from the resources and add the gradle wrapper
+// Useful for testing an intergration test project in a real environment
+class ExtractTestProject {
+	static void main(String[] args) {
+		if (args.length != 1) {
+			throw new IllegalArgumentException("Expected one argument: the project name")
+		}
+
+		def projectName = args[0]
+		def targetDirectory = new File("test/$projectName")
+		def sourceDirectory = new File("src/test/resources/projects/$projectName")
+
+		if (targetDirectory.exists()) {
+			targetDirectory.deleteDir()
+		}
+
+		copyDir(sourceDirectory, targetDirectory)
+		copyDir(new File("gradle"), new File(targetDirectory, "gradle"))
+		copyFile(new File("gradlew"), new File(targetDirectory, "gradlew"))
+		copyFile(new File("gradlew.bat"), new File(targetDirectory, "gradlew.bat"))
+
+		new File(targetDirectory, "settings.gradle").text = """
+            pluginManagement {
+                repositories {
+                    maven {
+                        name = 'Fabric'
+                        url = 'https://maven.fabricmc.net/'
+                    }
+                    mavenCentral()
+                    gradlePluginPortal()
+                    mavenLocal()
+                }
+            }
+            """
+
+		def buildGradle = new File(targetDirectory, "build.gradle")
+		buildGradle.text = buildGradle.text.replace("id 'fabric-loom'", "id 'fabric-loom' version '1.8.local'")
+	}
+
+	private static void copyDir(File source, File target) {
+		source.eachFileRecurse { file ->
+			if (file.isDirectory()) {
+				return
+			}
+
+			def relativePath = source.toPath().relativize(file.toPath())
+			def targetFile = new File(target, relativePath.toString())
+			copyFile(file, targetFile)
+		}
+	}
+
+	private static void copyFile(File source, File target) {
+		target.parentFile.mkdirs()
+		target.bytes = source.bytes
+		println("Copied $source to $target")
+	}
+}

src/test/resources/projects/splitSources/build.gradle

@@ -1,5 +1,5 @@
 plugins {
-	id 'fabric-loom' version '0.12.local'
+	id 'fabric-loom'
 	id 'maven-publish'
 }
 
@@ -24,13 +24,18 @@ loom {
 dependencies {
 	minecraft "com.mojang:minecraft:1.18.2"
 	mappings "net.fabricmc:yarn:1.18.2+build.1:v2"
-	modImplementation "net.fabricmc:fabric-loader:0.13.3"
+	modImplementation "net.fabricmc:fabric-loader:0.16.5"
+	testImplementation "net.fabricmc:fabric-loader-junit:0.16.5"
 
 	modImplementation "net.fabricmc.fabric-api:fabric-api:0.47.8+1.18.2"
 
 	shade "com.googlecode.json-simple:json-simple:1.1.1"
 }
 
+test {
+	useJUnitPlatform()
+}
+
 tasks.withType(JavaCompile).configureEach {
 	it.options.release = 17
 }

src/test/resources/projects/splitSources/src/client/resources/modid.client.mixins.json

@@ -1,7 +1,7 @@
 {
   "required": true,
   "minVersion": "0.8",
-  "package": "net.fabricmc.example.mixin",
+  "package": "net.fabricmc.example.client.mixin",
   "compatibilityLevel": "JAVA_17",
   "mixins": [
   ],

src/test/resources/projects/splitSources/src/test/java/net/fabricmc/example/test/ExampleModTest.java

@@ -0,0 +1,17 @@
+package net.fabricmc.example.test;
+
+import net.fabricmc.example.client.ExampleModClient;
+
+import net.minecraft.client.MinecraftClient;
+import org.junit.jupiter.api.Test;
+
+public class ExampleModTest {
+	@Test
+	void testClientClass() {
+		// Check we can compile against our own client code
+		ExampleModClient exampleModClient = new ExampleModClient();
+
+		// And minecrafts client code
+		MinecraftClient client = null;
+	}
+}