Add constexpr TaskTraits constructor.

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 47e9b81eeecd46877ffb4f71a42d173b04aa35b8..16774378e3b88f442c7f6805f4de430259c91577 100644
--- a/base/task_scheduler/task_traits.h
+++ b/base/task_scheduler/task_traits.h
@@ -10,6 +10,7 @@
 #include <iosfwd>
 
 #include "base/base_export.h"
+#include "base/task_scheduler/task_traits_details.h"
 #include "build/build_config.h"
 
 namespace base {
@@ -77,72 +78,92 @@ enum class TaskShutdownBehavior {
   BLOCK_SHUTDOWN,
 };
 
+// Tasks with this trait may block. This includes but is not limited to tasks
+// that wait on synchronous file I/O operations: read or write a file from disk,
+// interact with a pipe or a socket, rename or delete a file, enumerate files in
+// a directory, etc. This trait isn't required for the mere use of locks. For
+// tasks that block on base/ synchronization primitives, see the
+// WithBaseSyncPrimitives trait.
+struct MayBlock {};
+
+// Tasks with this trait will pass base::AssertWaitAllowed(), i.e. will be
+// allowed on the following methods :
+// - base::WaitableEvent::Wait
+// - base::ConditionVariable::Wait
+// - base::PlatformThread::Join
+// - base::PlatformThread::Sleep
+// - base::Process::WaitForExit
+// - base::Process::WaitForExitWithTimeout
+//
+// Tasks should generally not use these methods.
+//
+// Instead of waiting on a WaitableEvent or a ConditionVariable, put the work
+// that should happen after the wait in a callback and post that callback from
+// where the WaitableEvent or ConditionVariable would have been signaled. If
+// something needs to be scheduled after many tasks have executed, use
+// base::BarrierClosure.
+//
+// On Windows, join processes asynchronously using base::win::ObjectWatcher.
+//
+// MayBlock() must be specified in conjunction with this trait if and only if
+// removing usage of methods listed above in the labeled tasks would still
+// result in tasks that may block (per MayBlock()'s definition).
+//
+// In doubt, consult with //base/task_scheduler/OWNERS.
+struct WithBaseSyncPrimitives {};
+
 // Describes metadata for a single task or a group of tasks.
 class BASE_EXPORT TaskTraits {
  public:
-  // Constructs a default TaskTraits for tasks that
+  // Invoking this constructor without arguments produces TaskTraits that are
+  // appropriate for tasks that
   //     (1) don't block (ref. MayBlock() and WithBaseSyncPrimitives()),
   //     (2) prefer inheriting the current priority to specifying their own, and
   //     (3) can either block shutdown or be skipped on shutdown
   //         (TaskScheduler implementation is free to choose a fitting default).
-  // Tasks that require stricter guarantees and/or know the specific
-  // TaskPriority appropriate for them should highlight those by requesting
-  // explicit traits below.
-  TaskTraits();
-  TaskTraits(const TaskTraits& other) = default;
-  TaskTraits& operator=(const TaskTraits& other) = default;
-  ~TaskTraits();
-
-  // Tasks with this trait may block. This includes but is not limited to tasks
-  // that wait on synchronous file I/O operations: read or write a file from
-  // disk, interact with a pipe or a socket, rename or delete a file, enumerate
-  // files in a directory, etc. This trait isn't required for the mere use of
-  // locks. For tasks that block on base/ synchronization primitives, see
-  // WithBaseSyncPrimitives().
-  TaskTraits& MayBlock();
-
-  // Tasks with this trait will pass base::AssertWaitAllowed(), i.e. will be
-  // allowed on the following methods :
-  // - base::WaitableEvent::Wait
-  // - base::ConditionVariable::Wait
-  // - base::PlatformThread::Join
-  // - base::PlatformThread::Sleep
-  // - base::Process::WaitForExit
-  // - base::Process::WaitForExitWithTimeout
-  //
-  // Tasks should generally not use these methods.
-  //
-  // Instead of waiting on a WaitableEvent or a ConditionVariable, put the work
-  // that should happen after the wait in a callback and post that callback from
-  // where the WaitableEvent or ConditionVariable would have been signaled. If
-  // something needs to be scheduled after many tasks have executed, use
-  // base::BarrierClosure.
-  //
-  // Avoid creating threads. Instead, use
-  // base::Create(Sequenced|SingleTreaded)TaskRunnerWithTraits(). If a thread is
-  // really needed, make it non-joinable and add cleanup work at the end of the
-  // thread's main function (if using base::Thread, override Cleanup()).
-  //
-  // On Windows, join processes asynchronously using base::win::ObjectWatcher.
   //
-  // MayBlock() must be specified in conjunction with this trait if and only if
-  // removing usage of methods listed above in the labeled tasks would still
-  // result in tasks that may block (per MayBlock()'s definition).
+  // To get TaskTraits for tasks that require stricter guarantees and/or know
+  // the specific TaskPriority appropriate for them, provide arguments of type
+  // TaskPriority, TaskShutdownBehavior, MayBlock and/or WithBaseSyncPrimitives
+  // in any order to the constructor.
   //
-  // In doubt, consult with //base/task_scheduler/OWNERS.
-  TaskTraits& WithBaseSyncPrimitives();
+  // E.g.
+  // constexpr TaskTraits default_traits;
+  // constexpr TaskTraits user_visible_traits = {TaskPriority::USER_VISIBLE};
+  // constexpr TaskTraits user_visible_may_block_traits = {
+  //     TaskPriority::USER_VISIBLE, MayBlock()};
+  // constexpr TaskTraits other_user_visible_may_block_traits = {
+  //     MayBlock(), TaskPriority::USER_VISIBLE};

[gab, 2017/04/25 17:48:14]

Add base:: namespace in examples for ease of copy/paste.

[fdoray, 2017/04/26 17:20:33]

Done.

+  template <class... ArgTypes>
+  constexpr TaskTraits(const ArgTypes&... args)
+      : priority_(internal::GetValueFromArgList(
+            internal::CallFirstTag(),
+            internal::EnumArgGetter<base::TaskPriority,
+                                    base::TaskPriority::INHERITED>(),
+            args...)),
+        shutdown_behavior_(internal::GetValueFromArgList(
+            internal::CallFirstTag(),
+            internal::EnumArgGetter<
+                base::TaskShutdownBehavior,
+                base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN>(),
+            args...)),
+        may_block_(internal::GetValueFromArgList(
+            internal::CallFirstTag(),
+            internal::BooleanArgGetter<base::MayBlock>(),
+            args...)),
+        with_base_sync_primitives_(internal::GetValueFromArgList(
+            internal::CallFirstTag(),
+            internal::BooleanArgGetter<base::WithBaseSyncPrimitives>(),
+            args...)) {}
+
+  constexpr TaskTraits(const TaskTraits& other) = default;
+  TaskTraits& operator=(const TaskTraits& other) = default;
 
-  // Applies |priority| to tasks with these traits.
+  // Deprecated.

[gab, 2017/04/25 17:48:14]

// Deprecated. Prefer constexpr construction to builder paradigm as documented
above.

[fdoray, 2017/04/26 17:20:33]

Done.

   TaskTraits& WithPriority(TaskPriority priority);
-
-  // Applies |shutdown_behavior| to tasks with these traits.
   TaskTraits& WithShutdownBehavior(TaskShutdownBehavior shutdown_behavior);
-
-  // Returns true if tasks with these traits may block.
-  bool may_block() const { return may_block_; }
-
-  // Returns true if tasks with these traits may use base/ sync primitives.
-  bool with_base_sync_primitives() const { return with_base_sync_primitives_; }
+  TaskTraits& MayBlock();
+  TaskTraits& WithBaseSyncPrimitives();
 
   // Returns the priority of tasks with these traits.
   TaskPriority priority() const { return priority_; }
@@ -150,11 +171,17 @@ class BASE_EXPORT TaskTraits {
   // Returns the shutdown behavior of tasks with these traits.
   TaskShutdownBehavior shutdown_behavior() const { return shutdown_behavior_; }
 
+  // Returns true if tasks with these traits may block.
+  bool may_block() const { return may_block_; }

[etipdoray, 2017/04/24 20:22:02]

Accessors could be constexpr as well.

[fdoray, 2017/04/26 17:20:33]

Done.

+
+  // Returns true if tasks with these traits may use base/ sync primitives.
+  bool with_base_sync_primitives() const { return with_base_sync_primitives_; }
+
  private:
-  bool may_block_;
-  bool with_base_sync_primitives_;
   TaskPriority priority_;

[gab, 2017/04/25 17:48:15]

// TODO(fdoray): Make these const after refactoring away deprecated builder
pattern.

[fdoray, 2017/04/26 17:20:33]

Done.

   TaskShutdownBehavior shutdown_behavior_;
+  bool may_block_;
+  bool with_base_sync_primitives_;
 };
 
 // Returns string literals for the enums defined in this file. These methods