assertion-arguments: Detect error constructors passed as assertion messages

Fixes #178

Author: sindresorhus

docs/rules/assertion-arguments.md

@@ -28,6 +28,7 @@ test('1', t => {
 	t.is(value); // Not enough arguments
 	t.is(value, expected, message, extra); // Too many arguments
 	t.is(value, expected, false); // Assertion message is not a string
+	t.notThrows(() => { /* … */ }, TypeError); // Second argument should be a string message
 	t.plan('1'); // Argument is not a non-negative integer
 	t.plan(2.5); // Argument is not a non-negative integer
 	t.regex(/foo/, string); // First argument should be a string, not a regex

rules/assertion-arguments.js

@@ -333,20 +333,19 @@ const create = context => {
 
 				if (lastArgument.type === 'Identifier') {
 					const variable = findVariable(context.sourceCode.getScope(node), lastArgument);
-					if (!variable) {
-						return;
-					}
 
-					let value;
-					for (const reference of variable.references) {
-						value = reference.writeExpr ?? value;
+					let resolved;
+					if (variable) {
+						for (const reference of variable.references) {
+							resolved = reference.writeExpr ?? resolved;
+						}
 					}
 
-					if (!value) {
+					if (resolved) {
+						lastArgument = resolved;
+					} else if (!/^(?:[A-Z][a-z\d]*)*Error$/.test(lastArgument.name)) {
 						return;
 					}
-
-					lastArgument = value;
 				}
 
 				if (!isString(lastArgument)) {

test/assertion-arguments.js

@@ -509,6 +509,15 @@ ruleTester.run('assertion-arguments', rule, {
 		testCase(false, 't.fail({});', messageIsNotStringError),
 		testCase(false, 'let message = "ok"; message = false; t.assert(true, message);', messageIsNotStringError),
 
+		// Error constructors are not valid assertion messages
+		testCase(false, 't.notThrows(() => {}, TypeError);', messageIsNotStringError),
+		testCase(false, 't.notThrowsAsync(Promise.resolve(), TypeError);', messageIsNotStringError),
+		testCase(false, 't.notThrows(() => {}, Error);', messageIsNotStringError),
+		testCase(false, 't.notThrows(() => {}, RangeError);', messageIsNotStringError),
+		testCase(false, 't.fail(CustomError);', messageIsNotStringError),
+		testCase(false, 't.is(1, 2, TypeError);', messageIsNotStringError),
+		testCase(false, 't.throws(() => {}, null, TypeError);', messageIsNotStringError),
+
 		// Regex as first argument
 		testCase(false, 't.regex(/foo/, variable);', regexFirstError, {output: 't.regex(variable, /foo/);'}),
 		testCase(false, 't.notRegex(/foo/, variable);', regexFirstError, {output: 't.notRegex(variable, /foo/);'}),