No test files found even though one specified with --file

[gabegorelick]

Prerequisites

• Checked that your issue hasn't already been filed by cross-referencing issues with the faq label
• Checked next-gen ES issues and syntax problems by using the same environment and/or transpiler configuration without Mocha to ensure it isn't just a feature that actually isn't supported in the environment in question or a bug in your code.
• 'Smoke tested' the code to be tested by running it outside the real test suite to get a better sense of whether the problem is in the code under test, your usage of Mocha, or Mocha itself
• Ensured that there is no discrepancy between the locally and globally installed versions of Mocha. You can find them with: node node_modules/.bin/mocha --version(Local) and mocha --version(Global). We recommend that you not install Mocha globally.

Description

If mocha doesn't find any test files, it returns a No test files found error. This appears to happen even if --file is specified.

Steps to Reproduce

mocha --file foo.js --exclude '**/*.js'

Per the docs, "Files specified using --file are not affected" by --exclude, so this should work.

Expected behavior: foo.js is loaded

Actual behavior: No test files found error

Reproduces how often: 100%

Versions

• The output of mocha --version and node node_modules/.bin/mocha --version: 6.1.4
• The output of node --version: v8.15.1
• Your operating system
  • name and version: macOS
  • architecture (32 or 64-bit): 64bit
• Your shell (e.g., bash, zsh, PowerShell, cmd): zsh
• Your browser and version (if running browser tests): n/a
• Any third-party Mocha-related modules (and their versions): none
• Any code transpiler (e.g., TypeScript, CoffeeScript, Babel) being used (and its version): none

Additional Information

Use case for this is to run only a given set of files and override any "default" files loaded via positional arguments. For example, mocha --file test/foo.js --exclude '**/*.js' 'test/**/*.js'
would only run test/foo.js.

[gabegorelick]

I think this is the source of the issue:

mocha/lib/cli/run-helpers.js

Lines 142 to 168 in f3ed729

	if (!files.length) {
	// give full message details when only 1 file is missing
	const noneFoundMsg =
	unmatched.length === 1
	? `Error: No test files found: ${JSON.stringify(unmatched[0].pattern)}` // stringify to print escaped characters raw
	: 'Error: No test files found';
	console.error(ansi.red(noneFoundMsg));
	process.exit(1);
	} else {
	// print messages as an warning
	unmatched.forEach(warning => {
	console.warn(ansi.yellow(`Warning: ${warning.message}`));
	});
	}
	const fileArgs = file.map(filepath => path.resolve(filepath));
	files = files.map(filepath => path.resolve(filepath));
	// ensure we don't sort the stuff from fileArgs; order is important!
	if (sort) {
	files.sort();
	}
	// add files given through --file to be ran first
	files = fileArgs.concat(files);
	debug('files (in order): ', files);
	return files;

Note how files from --file is only added after the error is thrown.

[outsideris]

The previous comment that I deleted is caused by my misunderstanding. Sorry.