Output message when t.throws() fails (#1408)

lib/assert.js

@@ -41,7 +41,6 @@ function wrapAssertions(callbacks) {
 	const fail = callbacks.fail;
 
 	const noop = () => {};
-	const makeNoop = () => noop;
 	const makeRethrow = reason => () => {
 		throw reason;
 	};
@@ -174,7 +173,14 @@ function wrapAssertions(callbacks) {
 			if (promise) {
 				// Record stack before it gets lost in the promise chain.
 				const stack = getStack();
-				const intermediate = promise.then(makeNoop, makeRethrow).then(fn => test(fn, stack));
+				const intermediate = promise.then(value => {
+					throw new AssertionError({
+						assertion: 'throws',
+						message: 'Expected promise to be rejected, but it was resolved instead',
+						values: [formatAssertError.formatWithLabel('Resolved with:', value)]
+					});
+				}, reason => test(makeRethrow(reason), stack));
+
 				pending(this, intermediate);
 				// Don't reject the returned promise, even if the assertion fails.
 				return intermediate.catch(noop);

test/assert.js

@@ -12,16 +12,18 @@ const assertions = assert.wrapAssertions({
 		lastPassed = true;
 	},
 
-	pending() {},
+	pending(_, promise) {
+		promise.catch(err => {
+			lastFailure = err;
+		});
+	},
 
 	fail(_, error) {
 		lastFailure = error;
 	}
 });
 
-function failsWith(t, fn, subset) {
-	lastFailure = null;
-	fn();
+function assertFailure(t, subset) {
 	if (!lastFailure) {
 		t.fail('Expected assertion to fail');
 		return;
@@ -51,6 +53,19 @@ function failsWith(t, fn, subset) {
 	}
 }
 
+function failsWith(t, fn, subset) {
+	lastFailure = null;
+	fn();
+	assertFailure(t, subset);
+}
+
+function eventuallyFailsWith(t, promise, subset) {
+	lastFailure = null;
+	return promise.then(() => {
+		assertFailure(t, subset);
+	});
+}
+
 function fails(t, fn) {
 	lastFailure = null;
 	fn();
@@ -648,6 +663,14 @@ test('.throws() fails if passed a bad value', t => {
 	t.end();
 });
 
+test('promise .throws() fails when promise is resolved', t => {
+	return eventuallyFailsWith(t, assertions.throws(Promise.resolve('foo')), {
+		assertion: 'throws',
+		message: 'Expected promise to be rejected, but it was resolved instead',
+		values: [{label: 'Resolved with:', formatted: formatValue('foo')}]
+	});
+});
+
 test('.notThrows()', t => {
 	passes(t, () => {
 		assertions.notThrows(() => {});