Add base::TaskTraits::Override().

base/task_scheduler/task_traits.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_TASK_SCHEDULER_TASK_TRAITS_H_
 #define BASE_TASK_SCHEDULER_TASK_TRAITS_H_
 
 #include <stdint.h>
 
 #include <iosfwd>
@@ skipping 139 matching lines @@
   template <class... ArgTypes,
             class CheckArgumentsAreValid = internal::InitTypes<
                 decltype(ValidTrait(std::declval<ArgTypes>()))...>>
   constexpr TaskTraits(ArgTypes... args)
       : priority_set_explicitly_(
             internal::HasArgOfType<base::TaskPriority, ArgTypes...>::value),
         priority_(internal::GetValueFromArgList(
             internal::EnumArgGetter<base::TaskPriority,
                                     base::TaskPriority::USER_VISIBLE>(),
             args...)),
+        shutdown_behavior_set_explicitly_(
+            internal::HasArgOfType<base::TaskShutdownBehavior,
+                                   ArgTypes...>::value),
         shutdown_behavior_(internal::GetValueFromArgList(
             internal::EnumArgGetter<
                 base::TaskShutdownBehavior,
                 base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN>(),
             args...)),
         may_block_(internal::GetValueFromArgList(
             internal::BooleanArgGetter<base::MayBlock>(),
             args...)),
         with_base_sync_primitives_(internal::GetValueFromArgList(
             internal::BooleanArgGetter<base::WithBaseSyncPrimitives>(),
             args...)) {}
 
   constexpr TaskTraits(const TaskTraits& other) = default;
   TaskTraits& operator=(const TaskTraits& other) = default;
 
+  // Returns TaskTraits constructed by merging |left| and |right|. If a trait is
+  // specified in both |left| and |right|, the returned TaskTraits will have the
+  // value from |right|.
+  static constexpr TaskTraits Merge(const TaskTraits& left,

[gab, 2017/05/09 14:32:31]

s/Merge/Override/ as discussed.

[fdoray, 2017/05/09 14:58:33]

Done.

+                                    const TaskTraits& right) {
+    return TaskTraits(left, right);
+  }
+
   // Deprecated.  Prefer constexpr construction to builder paradigm as
   // documented above.
   // TODO(fdoray): Remove these methods. https://crbug.com/713683
   TaskTraits& WithPriority(TaskPriority priority);
   TaskTraits& WithShutdownBehavior(TaskShutdownBehavior shutdown_behavior);
   TaskTraits& MayBlock();
   TaskTraits& WithBaseSyncPrimitives();
 
   // Returns true if the priority was set explicitly.
   constexpr bool priority_set_explicitly() const {
@@ skipping 10 matching lines @@
 
   // Returns true if tasks with these traits may block.
   constexpr bool may_block() const { return may_block_; }
 
   // Returns true if tasks with these traits may use base/ sync primitives.
   constexpr bool with_base_sync_primitives() const {
     return with_base_sync_primitives_;
   }
 
  private:
+  constexpr TaskTraits(const TaskTraits& left, const TaskTraits& right)
+      : priority_set_explicitly_(left.priority_set_explicitly_ ||
+                                 right.priority_set_explicitly_),
+        priority_(right.priority_set_explicitly_ ? right.priority_
+                                                 : left.priority_),
+        shutdown_behavior_set_explicitly_(
+            left.shutdown_behavior_set_explicitly_ ||
+            right.shutdown_behavior_set_explicitly_),
+        shutdown_behavior_(right.shutdown_behavior_set_explicitly_
+                               ? right.shutdown_behavior_
+                               : left.shutdown_behavior_),
+        may_block_(left.may_block_ || right.may_block_),
+        with_base_sync_primitives_(left.with_base_sync_primitives_ ||
+                                   right.with_base_sync_primitives_) {}
+
   // TODO(fdoray): Make these const after refactoring away deprecated builder
   // pattern.

[gab, 2017/05/09 14:32:31]

drive-by thought:

Actually, this TODO will not be possible because of the need to support
operator=() for the STL.

But you'll still be able to say it's an immutable object in its meta-comment per
all its public methods being const (once builders are removed).

[fdoray, 2017/05/09 14:58:33]

Acknowledged. TODO removed by upcoming CL.

   bool priority_set_explicitly_;
   TaskPriority priority_;
+  bool shutdown_behavior_set_explicitly_;
   TaskShutdownBehavior shutdown_behavior_;
   bool may_block_;
   bool with_base_sync_primitives_;
 };
 
 // Returns string literals for the enums defined in this file. These methods
 // should only be used for tracing and debugging.
 BASE_EXPORT const char* TaskPriorityToString(TaskPriority task_priority);
 BASE_EXPORT const char* TaskShutdownBehaviorToString(
     TaskShutdownBehavior task_priority);
 
 // Stream operators so that the enums defined in this file can be used in
 // DCHECK and EXPECT statements.
 BASE_EXPORT std::ostream& operator<<(std::ostream& os,
                                      const TaskPriority& shutdown_behavior);
 BASE_EXPORT std::ostream& operator<<(
     std::ostream& os,
     const TaskShutdownBehavior& shutdown_behavior);
 
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_TRAITS_H_