util: support classes in util.deprecate()

Classes cannot be instantiated without new, but util.deprecate()
uses Function.prototype.apply(). This commit uses new.target to
detect constructor calls, allowing classes to be deprecated.

PR-URL: #7690
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/util.md

@@ -46,7 +46,7 @@ environment variable. For example: `NODE_DEBUG=fs,net,tls`.
 
 ## util.deprecate(function, string)
 
-The `util.deprecate()` method wraps the given `function` in such a way that
+The `util.deprecate()` method wraps the given `function` or class in such a way that
 it is marked as deprecated.
 
 ```js

lib/internal/util.js

@@ -57,9 +57,18 @@ exports._deprecate = function(fn, msg) {
   var warned = false;
   function deprecated() {
     warned = exports.printDeprecationMessage(msg, warned, deprecated);
+    if (new.target) {
+      return Reflect.construct(fn, arguments, new.target);
+    }
     return fn.apply(this, arguments);
   }
 
+  // The wrapper will keep the same prototype as fn to maintain prototype chain
+  Object.setPrototypeOf(deprecated, fn);
+  if (fn.prototype) {
+    Object.setPrototypeOf(deprecated.prototype, fn.prototype);
+  }
+
   return deprecated;
 };
 

test/fixtures/deprecated-userland-class.js

@@ -0,0 +1,12 @@
+const util = require('util');
+const assert = require('assert');
+
+class deprecatedClass {
+}
+
+const deprecated = util.deprecate(deprecatedClass, 'deprecatedClass is deprecated.');
+
+const instance = new deprecated();
+
+assert(instance instanceof deprecated);
+assert(instance instanceof deprecatedClass);

test/fixtures/deprecated-userland-subclass.js

@@ -0,0 +1,19 @@
+const util = require('util');
+const assert = require('assert');
+
+class deprecatedClass {
+}
+
+const deprecated = util.deprecate(deprecatedClass, 'deprecatedClass is deprecated.');
+
+class subclass extends deprecated {
+  constructor() {
+    super();
+  }
+}
+
+const instance = new subclass();
+
+assert(instance instanceof subclass);
+assert(instance instanceof deprecated);
+assert(instance instanceof deprecatedClass);

test/sequential/test-deprecation-flags.js

@@ -5,8 +5,14 @@ const execFile = require('child_process').execFile;
 const depmod = require.resolve(common.fixturesDir + '/deprecated.js');
 const node = process.execPath;
 
-const depUserland =
-    require.resolve(common.fixturesDir + '/deprecated-userland-function.js');
+const depUserlandFunction =
+  require.resolve(common.fixturesDir + '/deprecated-userland-function.js');
+
+const depUserlandClass =
+  require.resolve(common.fixturesDir + '/deprecated-userland-class.js');
+
+const depUserlandSubClass =
+  require.resolve(common.fixturesDir + '/deprecated-userland-subclass.js');
 
 const normal = [depmod];
 const noDep = ['--no-deprecation', depmod];
@@ -39,10 +45,22 @@ execFile(node, traceDep, function(er, stdout, stderr) {
   console.log('trace ok');
 });
 
-execFile(node, [depUserland], function(er, stdout, stderr) {
+execFile(node, [depUserlandFunction], function(er, stdout, stderr) {
   console.error('normal: testing deprecated userland function');
   assert.equal(er, null);
   assert.equal(stdout, '');
   assert(/deprecatedFunction is deprecated/.test(stderr));
   console.error('normal: ok');
 });
+
+execFile(node, [depUserlandClass], function(er, stdout, stderr) {
+  assert.strictEqual(er, null);
+  assert.strictEqual(stdout, '');
+  assert(/deprecatedClass is deprecated/.test(stderr));
+});
+
+execFile(node, [depUserlandSubClass], function(er, stdout, stderr) {
+  assert.strictEqual(er, null);
+  assert.strictEqual(stdout, '');
+  assert(/deprecatedClass is deprecated/.test(stderr));
+});