Fix: Throw better error when watch parameters are invalid (fixes #1002)

index.js

@@ -13,6 +13,13 @@ Gulp.prototype.src = vfs.src;
 Gulp.prototype.dest = vfs.dest;
 Gulp.prototype.symlink = vfs.symlink;
 Gulp.prototype.watch = function(glob, opt, task) {
+  if (typeof opt === 'string' || typeof task === 'string' ||
+    Array.isArray(opt) || Array.isArray(task)) {
+    throw new Error('watching ' + glob + ': watch task has to be ' +
+      'a function (optionally generated by using gulp.parallel ' +
+      'or gulp.series)');
+  }
+
   if (typeof opt === 'function') {
     task = opt;
     opt = null;

test/watch.js

@@ -126,5 +126,29 @@ describe('gulp', function() {
       updateTempFile(tempFile);
     });
 
+    it('should throw an error: passed parameter (string) is not a function', function(done) {
+      var tempFile = path.join(outpath, 'empty.txt');
+
+      createTempFile(tempFile);
+      try {
+        gulp.watch(tempFile, 'task1');
+      } catch (err) {
+        err.message.should.equal('watching ' + tempFile +  ': watch task has to be a function (optionally generated by using gulp.parallel or gulp.series)');
+        done();
+      }
+    });
+
+    it('should throw an error: passed parameter (array) is not a function', function(done) {
+      var tempFile = path.join(outpath, 'empty.txt');
+
+      createTempFile(tempFile);
+      try {
+        gulp.watch(tempFile, ['task1']);
+      } catch (err) {
+        err.message.should.equal('watching ' + tempFile +  ': watch task has to be a function (optionally generated by using gulp.parallel or gulp.series)');
+        done();
+      }
+    });
+
   });
 });