CLI: error on a non-existent oldfile/newfile

cli/src/main/scala/com/typesafe/tools/mima/cli/MimaCli.scala

@@ -19,6 +19,10 @@ case class Main(
     val newBin = newBinOpt.getOrElse(
       throw new IllegalArgumentException("New binary was not specified")
     )
+    if (!oldBin.exists())
+      throw new IllegalArgumentException(s"oldfile does not exist: $oldBin")
+    if (!newBin.exists())
+      throw new IllegalArgumentException(s"newfile does not exist: $newBin")
     val problems = new MiMaLib(classpath)
       .collectProblems(oldBin, newBin, Nil)
       .flatMap(formatter.formatProblem)

cli/src/test/scala/com/typesafe/tools/mima/cli/MainSpec.scala

@@ -0,0 +1,31 @@
+package com.typesafe.tools.mima.cli
+
+import java.io.File
+import java.nio.file.Files
+
+final class MainSpec extends munit.FunSuite {
+  val existingDir = new File(".") // exists but contributes no classes (like a POM-only module)
+  val missing     = new File("does-not-exist.jar")
+
+  test("a non-existent oldfile is an error") {
+    val ex = intercept[IllegalArgumentException] {
+      Main(oldBinOpt = Some(missing), newBinOpt = Some(existingDir)).run()
+    }
+    assert(ex.getMessage.contains("oldfile does not exist"), ex.getMessage)
+  }
+
+  test("a non-existent newfile is an error") {
+    val ex = intercept[IllegalArgumentException] {
+      Main(oldBinOpt = Some(existingDir), newBinOpt = Some(missing)).run()
+    }
+    assert(ex.getMessage.contains("newfile does not exist"), ex.getMessage)
+  }
+
+  test("existing but class-less paths are not an error (POM-only-style)") {
+    val empty = Files.createTempDirectory("mima-cli-test").toFile
+    try {
+      val problems = Main(oldBinOpt = Some(empty), newBinOpt = Some(empty)).run()
+      assertEquals(problems, 0)
+    } finally empty.delete()
+  }
+}