Try to fail test if assertions are added after test finishes

Though currently this error is likely to get lost unless there is a
pending assertion or `test.cb()` is used.

lib/test.js

@@ -101,6 +101,7 @@ class Test {
 		this.calledEnd = false;
 		this.duration = null;
 		this.endCallbackFinisher = null;
+		this.finishing = false;
 		this.pendingAssertions = [];
 		this.planCount = null;
 		this.startedAt = 0;
@@ -142,15 +143,27 @@ class Test {
 	}
 
 	countPassedAssertion() {
+		if (this.finishing) {
+			this.saveFirstError(new Error('Assertion passed, but test has already ended'));
+		}
+
 		this.assertCount++;
 	}
 
 	addPendingAssertion(promise) {
+		if (this.finishing) {
+			this.saveFirstError(new Error('Assertion passed, but test has already ended'));
+		}
+
 		this.assertCount++;
 		this.pendingAssertions.push(promise);
 	}
 
 	addFailedAssertion(error) {
+		if (this.finishing) {
+			this.saveFirstError(new Error('Assertion failed, but test has already ended'));
+		}
+
 		this.assertCount++;
 		this.saveFirstError(error);
 	}
@@ -265,12 +278,13 @@ class Test {
 	}
 
 	finish() {
+		this.finishing = true;
+		this.verifyPlan();
+
 		if (this.pendingAssertions.length === 0) {
-			return this.finishImmediately();
+			return this.completeFinish();
 		}
 
-		this.verifyPlan();
-
 		// Consume errors, ensuring there are no unhandled rejections.
 		const consumedErrors = Promise.all(this.pendingAssertions)
 			.catch(err => this.saveFirstError(err));
@@ -281,9 +295,7 @@ class Test {
 		}
 
 		// Finish after potential errors from pending assertions have been consumed.
-		// Note that the plan must be verified again in case a new assertion was
-		// added.
-		return consumedErrors.then(() => this.finishImmediately());
+		return consumedErrors.then(() => this.completeFinish());
 	}
 
 	finishPromised() {
@@ -292,11 +304,6 @@ class Test {
 		});
 	}
 
-	finishImmediately() {
-		this.verifyPlan();
-		return this.completeFinish();
-	}
-
 	completeFinish() {
 		this.duration = globals.now() - this.startedAt;
 		globals.clearTimeout(this.timeoutHandle);

test/test.js

@@ -508,21 +508,40 @@ test('multiple resolving and rejecting promises passed to t.throws/t.notThrows',
 	});
 });
 
-test('number of assertions matches t.plan when the test exits, but before all promises resolve another is added', t => {
+test('number of assertions matches t.plan when the test exits, but before all pending assertions resolve another is added', t => {
 	let result;
 	ava(a => {
 		a.plan(2);
 		a.throws(delay.reject(10, new Error('foo')), 'foo');
 		a.notThrows(delay(10), 'foo');
 		setTimeout(() => {
-			a.throws(Promise.reject(new Error('foo')), 'foo');
+			a.pass();
+		}, 5);
+	}, null, r => {
+		result = r;
+	}).run().then(passed => {
+		t.is(passed, false);
+		t.match(result.reason.message, /Assertion passed, but test has already ended/);
+		t.is(result.reason.name, 'Error');
+		t.end();
+	});
+});
+
+test('number of assertions matches t.plan when the test exits, but before all pending assertions resolve, a failing assertion is added', t => {
+	let result;
+	ava(a => {
+		a.plan(2);
+		a.throws(delay.reject(10, new Error('foo')), 'foo');
+		a.notThrows(delay(10), 'foo');
+		setTimeout(() => {
+			a.fail();
 		}, 5);
 	}, null, r => {
 		result = r;
 	}).run().then(passed => {
 		t.is(passed, false);
-		t.is(result.reason.assertion, 'plan');
-		t.is(result.reason.operator, '===');
+		t.match(result.reason.message, /Assertion failed, but test has already ended/);
+		t.is(result.reason.name, 'Error');
 		t.end();
 	});
 });