fix negative number handling (closes vercel#52)

index.js

@@ -100,7 +100,21 @@ function arg(opts, {argv = process.argv.slice(2), permissive = false, stopAtPosi
 				if (isFlag) {
 					result[argName] = type(true, argName, result[argName]);
 				} else if (argStr === undefined) {
-					if (argv.length < i + 2 || (argv[i + 1].length > 1 && argv[i + 1][0] === '-')) {
+					if (
+						argv.length < i + 2 ||
+						(
+							argv[i + 1].length > 1 &&
+							(argv[i + 1][0] === '-') &&
+							!(
+								argv[i + 1].match(/^-?\d*(\.(?=\d))?\d*$/) &&
+								(
+									type === Number ||
+									// eslint-disable-next-line no-undef
+									(typeof BigInt !== 'undefined' && type === BigInt)
+								)
+							)
+						)
+					) {
 						const extended = originalArgName === argName ? '' : ` (alias for ${argName})`;
 						throw new Error(`Option requires argument: ${originalArgName}${extended}`);
 					}

test.js

@@ -347,3 +347,53 @@ test('should stop parsing early with permissive', () => {
 		'-d': 2
 	});
 });
+
+test('should parse negative numbers (GNU equals form)', () => {
+	const argv = ['--int=-5'];
+
+	const result = arg({
+		'--int': Number
+	}, {
+		argv
+	});
+
+	expect(result).to.deep.equal({
+		_: [],
+		'--int': -5
+	});
+});
+
+test('should parse negative numbers (separate argument form)', () => {
+	const argv = ['--int', '-5'];
+
+	const result = arg({
+		'--int': Number
+	}, {
+		argv
+	});
+
+	expect(result).to.deep.equal({
+		_: [],
+		'--int': -5
+	});
+});
+
+test('should error if numeric type is followed by non-negative, non-argument', () => {
+	const argv = ['--int', '-abc'];
+
+	expect(() => arg({
+		'--int': Number
+	}, {
+		argv
+	})).to.throw(Error, 'Option requires argument: --int');
+});
+
+test('should error if negative numeric argument is passed to non-negative argument', () => {
+	const argv = ['--str', '-15'];
+
+	expect(() => arg({
+		'--str': String
+	}, {
+		argv
+	})).to.throw(Error, 'Option requires argument: --str');
+});