process: cast promise rejection reason to string

The unhandled promise rejection warning uses a template literal and
prints the reason a promise was rejected. If rejecting with a symbol,
the symbol failed to convert to a string and the process crashed. Now,
symbols are casted to strings and the process does not crash.

Fixes: #11637
PR-URL: #11640
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/internal/process/promises.js

@@ -57,7 +57,7 @@ function setupPromises(scheduleMicrotasks) {
 
   function emitWarning(uid, reason) {
     const warning = new Error('Unhandled promise rejection ' +
-                              `(rejection id: ${uid}): ${reason}`);
+                              `(rejection id: ${uid}): ${String(reason)}`);
     warning.name = 'UnhandledPromiseRejectionWarning';
     warning.id = uid;
     if (reason instanceof Error) {

test/parallel/test-promises-unhandled-symbol-rejections.js

@@ -0,0 +1,18 @@
+'use strict';
+const common = require('../common');
+
+const expectedDeprecationWarning = 'Unhandled promise rejections are ' +
+                                   'deprecated. In the future, promise ' +
+                                   'rejections that are not handled will ' +
+                                   'terminate the Node.js process with a ' +
+                                   'non-zero exit code.';
+const expectedPromiseWarning = 'Unhandled promise rejection (rejection id: ' +
+                               '1): Symbol()';
+
+common.expectWarning({
+  DeprecationWarning: expectedDeprecationWarning,
+  UnhandledPromiseRejectionWarning: expectedPromiseWarning,
+});
+
+// ensure this doesn't crash
+Promise.reject(Symbol());