Yield error if no files were found even without CLI arguments

lib/elm-files.js

@@ -53,6 +53,9 @@ async function getProjectFiles(options, elmSyntaxVersion) {
   const emptyDirectories = filesInDirectories.filter(
     ({files}) => files.length === 0
   );
+  const elmFilesToRead = [
+    ...new Set(filesInDirectories.flatMap((directory) => directory.files))
+  ];
 
   // TODO(@jfmengels): Have a different message depending on how directories were chosen (CLI directories, application, package).
   if (options.directoriesToAnalyze.length > 0 && emptyDirectories.length > 0) {
@@ -65,6 +68,12 @@ ${emptyDirectories.map(({directory}) => `- ${directory}`).join('\n')}
 When I can’t find files in some of the directories, I’m assuming that you
 misconfigured the CLI’s arguments.`
     );
+  } else if (elmFilesToRead.length === 0) {
+    throw new ErrorMessage.CustomError(
+      'NO FILES FOUND',
+      `I could not find any files in this project. I looked in these folders:
+${sourceDirectories.map((directory) => `- ${directory}`).join('\n')}`
+    );
   }
 
   Debug.log(`Parsing using stil4m/elm-syntax v${elmSyntaxVersion}`);
@@ -73,9 +82,6 @@ misconfigured the CLI’s arguments.`
   Benchmark.start(options, 'parse/fetch parsed files');
   const elmParserPath = options.elmParserPath(elmSyntaxVersion);
   elmParser.prepareWorkers();
-  const elmFilesToRead = [
-    ...new Set(filesInDirectories.flatMap((directory) => directory.files))
-  ];
   const elmFiles = await Promise.all(
     elmFilesToRead.map(
       async (filePath) =>

test/project-empty/elm.json

@@ -0,0 +1,24 @@
+{
+    "type": "application",
+    "source-directories": [
+        "src"
+    ],
+    "elm-version": "0.19.1",
+    "dependencies": {
+        "direct": {
+            "elm/browser": "1.0.2",
+            "elm/core": "1.0.5",
+            "elm/html": "1.0.0"
+        },
+        "indirect": {
+            "elm/json": "1.1.3",
+            "elm/time": "1.0.0",
+            "elm/url": "1.0.0",
+            "elm/virtual-dom": "1.0.3"
+        }
+    },
+    "test-dependencies": {
+        "direct": {},
+        "indirect": {}
+    }
+}

test/review.test.js

@@ -123,6 +123,14 @@ test('Running on project with unknown file', async () => {
   expect(output).toMatchFile(testName('run-with-unknown-target'));
 });
 
+test('Running on project with no files', async () => {
+  const output = await TestCli.runAndExpectError(
+    ['--config=../project-with-errors/review'],
+    {project: 'project-empty'}
+  );
+  expect(output).toMatchFile(testName('run-with-empty-project'));
+});
+
 test('Running on project with a directory ending in .elm (without arg)', async () => {
   const output = await TestCli.run(
     ['--config', '../config-that-triggers-no-errors'],

test/snapshots/review/run-with-empty-project.txt

@@ -0,0 +1,6 @@
+-- NO FILES FOUND --------------------------------------------------------------
+
+I could not find any files in this project. I looked in these folders:
+- <local-path>/test/project-empty/src
+- <local-path>/test/project-empty/tests
+