Ensure main classes from .scala inputs take precedence before main classes found in JARs added to the classpath

Author: Gedochao

modules/build/src/main/scala/scala/build/Build.scala

@@ -59,13 +59,14 @@ object Build {
     def outputOpt: Some[os.Path]          = Some(output)
     def dependencyClassPath: Seq[os.Path] = sources.resourceDirs ++ artifacts.classPath
     def fullClassPath: Seq[os.Path]       = Seq(output) ++ dependencyClassPath
+    private lazy val mainClassesFoundInProject: Seq[String] = MainClass.find(output).sorted
+    private lazy val mainClassesFoundOnExtraClasspath: Seq[String] =
+      options.classPathOptions.extraClassPath.flatMap(MainClass.find).sorted
+    private lazy val mainClassesFoundInUserExtraDependencies: Seq[String] =
+      artifacts.jarsForUserExtraDependencies.flatMap(MainClass.findInDependency).sorted
     def foundMainClasses(): Seq[String] = {
-      val found =
-        MainClass.find(output).sorted ++
-          options.classPathOptions.extraClassPath.flatMap(MainClass.find).sorted
-      if (inputs.isEmpty && found.isEmpty)
-        artifacts.jarsForUserExtraDependencies.flatMap(MainClass.findInDependency).sorted
-      else found
+      val found = mainClassesFoundInProject ++ mainClassesFoundOnExtraClasspath
+      if inputs.isEmpty && found.isEmpty then mainClassesFoundInUserExtraDependencies else found
     }
     def retainedMainClass(
       mainClasses: Seq[String],
@@ -121,8 +122,11 @@ object Build {
         }
 
       val filteredMainClasses =
-        mainClasses.filter(!scriptInferredMainClasses.contains(_))
-      if (filteredMainClasses.length == 1) {
+        mainClasses
+          .filterNot(scriptInferredMainClasses.contains(_))
+          .filterNot(mainClassesFoundOnExtraClasspath.contains(_))
+          .filterNot(mainClassesFoundInUserExtraDependencies.contains(_))
+      if filteredMainClasses.length == 1 then {
         val pickedMainClass = filteredMainClasses.head
         if (scriptInferredMainClasses.nonEmpty) {
           val firstScript   = scriptInferredMainClasses.head
@@ -139,8 +143,7 @@ object Build {
         }
         Right(pickedMainClass)
       }
-      else
-        Left(mainClasses)
+      else Left(mainClasses)
     }
     def retainedMainClassOpt(
       mainClasses: Seq[String],

modules/integration/src/test/scala/scala/cli/integration/RunTestDefinitions.scala

@@ -2303,4 +2303,37 @@ abstract class RunTestDefinitions
         expect(res.out.trim() == expectedOutput)
       }
   }
+
+  test(
+    "prioritise main class in a .scala file over main classes in dependencies on the classpath"
+  ) {
+    val expectedMessage = "Main"
+    TestInputs(
+      os.rel / "Main.scala" -> s"""object Main extends App { println("$expectedMessage") }"""
+    )
+      .fromRoot { root =>
+        val localCache        = root / "local-cache"
+        val dependencyVersion = "42.7.4"
+        val csRes = os.proc(
+          TestUtil.cs,
+          "fetch",
+          "--cache",
+          localCache,
+          s"org.postgresql:postgresql:$dependencyVersion"
+        )
+          .call(cwd = root)
+        val dependencyJar = csRes.out.trim().linesIterator.toSeq.head
+
+        // pass classpath via -cp
+        val res = os.proc(TestUtil.cli, "run", ".", extraOptions, "-cp", dependencyJar)
+          .call(cwd = root)
+        expect(res.out.trim() == expectedMessage)
+
+        // pass classpath via args file
+        os.write(root / "args.txt", s"-cp $dependencyJar")
+        val res2 = os.proc(TestUtil.cli, "run", ".", extraOptions, "@args.txt")
+          .call(cwd = root)
+        expect(res2.out.trim() == expectedMessage)
+      }
+  }
 }