Ensure CLI receives file paths

Fixes #2158.

lib/cli.js

@@ -1,5 +1,6 @@
 'use strict';
 const path = require('path');
+const fs = require('fs');
 const del = require('del');
 const updateNotifier = require('update-notifier');
 const figures = require('figures');
@@ -213,6 +214,24 @@ exports.run = async () => { // eslint-disable-line complexity
 
 	const match = arrify(conf.match);
 	const resolveTestsFrom = cli.input.length === 0 ? projectDir : process.cwd();
+
+	const files = cli.input.map(file => path.relative(resolveTestsFrom, path.resolve(process.cwd(), file)));
+
+	for (const file of cli.input) {
+		try {
+			const stats = fs.statSync(file);
+			if (!stats.isFile()) {
+				exit(`${file} is not a test file.`);
+			}
+		} catch (error) {
+			if (error.code === 'ENOENT') {
+				exit(`${file} does not exist.`);
+			} else {
+				exit(`Error accessing ${file}\n\n${chalk.gray((error && error.stack) || error)}`);
+			}
+		}
+	}
+
 	const api = new Api({
 		babelConfig,
 		cacheEnabled: conf.cache !== false,
@@ -268,8 +287,6 @@ exports.run = async () => { // eslint-disable-line complexity
 		});
 	});
 
-	const files = cli.input.map(file => path.relative(resolveTestsFrom, path.resolve(process.cwd(), file)));
-
 	if (conf.watch) {
 		const watcher = new Watcher({
 			api,

test/integration/assorted.js

@@ -27,6 +27,22 @@ test('timeout', t => {
 // 	}, 2000);
 // });
 
+test('Should throw error if passed file does not exist', t => {
+	execCli('no-such-file.js', (err, e, stdout) => {
+		t.ok(err);
+		t.match(stdout, /no-such-file\.js does not exist\./);
+		t.end();
+	});
+});
+
+test('Should throw error if passed file is a directory', t => {
+	execCli('ava-paths', (err, e, stdout) => {
+		t.ok(err);
+		t.match(stdout, /ava-paths is not a test file\./);
+		t.end();
+	});
+});
+
 test('include anonymous functions in error reports', t => {
 	execCli('error-in-anonymous-function.js', (err, stdout) => {
 		t.ok(err);