Add base::TaskTraits::Override().

base/task_scheduler/task_traits.h

Index: base/task_scheduler/task_traits.h
diff --git a/base/task_scheduler/task_traits.h b/base/task_scheduler/task_traits.h
index cd1d7ccf3c674a56ff6fc80a94b1d621f2b94ad5..f8cc30ebf7898dd95db7637d7e014a8ef9f876d0 100644
--- a/base/task_scheduler/task_traits.h
+++ b/base/task_scheduler/task_traits.h
@@ -157,6 +157,9 @@ class BASE_EXPORT TaskTraits {
             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,
@@ -172,6 +175,14 @@ class BASE_EXPORT TaskTraits {
   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
@@ -202,10 +213,26 @@ class BASE_EXPORT TaskTraits {
   }
 
  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_;