Convert generator-mw with deprecation warning

Readme.md

@@ -88,9 +88,24 @@ app.use(co.wrap(function *(ctx, next) {
 }));
 ```
 
-### Old signature middleware (v1.x)
+### Old signature middleware (v1.x) - Deprecated
 
-If you want to use old signature or be compatible with old middleware, you must use [koa-convert](https://github.com/gyson/koa-convert) to convert legacy generator middleware to promise middleware.
+**Old signature middleware (v1.x) support will be removed in v3**
+
+Koa v2.x will try to convert legacy signature, generator middleware on `app.use`, using [koa-convert](https://github.com/koajs/convert).
+It is however recommended that you choose to migrate all v1.x middleware as soon as possible.
+
+```js
+// Koa will convert
+app.use(function *(next) {
+  const start = new Date();
+  yield next;
+  const ms = new Date() - start;
+  console.log(`${this.method} ${this.url} - ${ms}ms`);
+});
+```
+
+You could do it manually as well, in which case Koa will not convert.
 
 ```js
 const convert = require('koa-convert');
@@ -103,7 +118,6 @@ app.use(convert(function *(next) {
 }));
 ```
 
-
 ## Babel setup
 For Node 4.0 and Babel 6.0 you can setup like this
 

lib/application.js

@@ -21,6 +21,8 @@ const assert = require('assert');
 const Stream = require('stream');
 const http = require('http');
 const only = require('only');
+const convert = require('koa-convert');
+const deprecate = require('depd')('koa');
 
 /**
  * Expose `Application` class.
@@ -93,14 +95,21 @@ module.exports = class Application extends Emitter {
   /**
    * Use the given middleware `fn`.
    *
+   * Old-style middleware will be converted.
+   *
    * @param {Function} fn
    * @return {Application} self
    * @api public
    */
 
   use(fn) {
     if (typeof fn !== 'function') throw new TypeError('middleware must be a function!');
-    if (isGeneratorFunction(fn)) throw new TypeError('Support for generators has been removed. See the documentation for examples of how to convert old middleware https://github.com/koajs/koa/tree/v2.x#old-signature-middleware-v1x');
+    if (isGeneratorFunction(fn)) {
+      deprecate('Support for generators will been removed in v3. ' +
+                'See the documentation for examples of how to convert old middleware ' +
+                'https://github.com/koajs/koa/tree/v2.x#old-signature-middleware-v1x');
+      fn = convert(fn);
+    }
     debug('use %s', fn._name || fn.name || '-');
     this.middleware.push(fn);
     return this;

package.json

@@ -24,6 +24,7 @@
     "cookies": "~0.6.1",
     "debug": "*",
     "delegates": "^1.0.0",
+    "depd": "^1.1.0",
     "destroy": "^1.0.3",
     "error-inject": "~1.0.0",
     "escape-html": "~1.0.1",
@@ -32,6 +33,7 @@
     "http-errors": "^1.2.8",
     "is-generator-function": "^1.0.3",
     "koa-compose": "^3.0.0",
+    "koa-convert": "^1.2.0",
     "koa-is-json": "^1.0.0",
     "mime-types": "^2.0.7",
     "on-finished": "^2.1.0",

test/application/use.js

@@ -2,6 +2,7 @@
 'use strict';
 
 const request = require('supertest');
+const assert = require('assert');
 const Koa = require('../..');
 
 describe('app.use(fn)', () => {
@@ -42,6 +43,43 @@ describe('app.use(fn)', () => {
       });
   });
 
+  it('should compose mixed middleware', done => {
+    process.once('deprecation', () => {}); // silence deprecation message
+    const app = new Koa();
+    const calls = [];
+
+    app.use((ctx, next) => {
+      calls.push(1);
+      return next().then(() => {
+        calls.push(6);
+      });
+    });
+
+    app.use(function *(next){
+      calls.push(2);
+      yield next;
+      calls.push(5);
+    });
+
+    app.use((ctx, next) => {
+      calls.push(3);
+      return next().then(() => {
+        calls.push(4);
+      });
+    });
+
+    const server = app.listen();
+
+    request(server)
+      .get('/')
+      .expect(404)
+      .end(err => {
+        if (err) return done(err);
+        calls.should.eql([1, 2, 3, 4, 5, 6]);
+        done();
+      });
+  });
+
   // https://github.com/koajs/koa/pull/530#issuecomment-148138051
   it('should catch thrown errors in non-async functions', done => {
     const app = new Koa();
@@ -54,16 +92,33 @@ describe('app.use(fn)', () => {
       .end(done);
   });
 
+  it('should accept both generator and function middleware', done => {
+    process.once('deprecation', () => {}); // silence deprecation message
+    const app = new Koa();
+
+    app.use((ctx, next) => { return next(); });
+    app.use(function *(next){ this.body = 'generator'; });
+
+    request(app.listen())
+      .get('/')
+      .expect(200)
+      .expect('generator', done);
+  });
+
   it('should throw error for non function', () => {
     const app = new Koa();
 
     [null, undefined, 0, false, 'not a function'].forEach(v => (() => app.use(v)).should.throw('middleware must be a function!'));
   });
 
-  it('should throw error for generator', () => {
-    const app = new Koa();
+  it('should output deprecation message for generator functions', done => {
+    process.once('deprecation', message => {
+      assert(/Support for generators will been removed/.test(message));
+      done();
+    });
 
-    (() => app.use(function *(){})).should.throw(/.+/);
+    const app = new Koa();
+    app.use(function *(){});
   });
 
   it('should throw error for non function', () => {