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..84499d550468837e95f923d782116e854641ca64 100644
--- a/base/task_scheduler/task_traits.h
+++ b/base/task_scheduler/task_traits.h
@@ -8,6 +8,7 @@
 #include <stdint.h>
 
 #include <iosfwd>
+#include <utility>
 
 #include "base/base_export.h"
 #include "build/build_config.h"
@@ -77,72 +78,159 @@ 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
+// WithBaseSyncPrimitives().
+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.
+//
+// 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).
+//
+// In doubt, consult with //base/task_scheduler/OWNERS.
+struct WithBaseSyncPrimitives {};
+
+namespace internal {
+
+// When the following call is made:
+//    GetTraitFromArgList(CallFirstTag(), GetterType(), args...);
+// The compiler prefers using the 3rd overload of GetTraitFromArgList(), because
+// the type of the first argument matches exactly. That overload uses
+// Getter::operator()(FirstArgType) to transform |first_arg| into a value that
+// can be used to initialize a member of TaskTraits. If
+// Getter::operator()(FirstArgType) isn't defined, the compiler uses the 1st or
+// the 2nd overload instead (depending on whether |args| is empty). The 1st
+// overload returns a default value. The 2nd overload discards |first_arg| and
+// calls back GetTraitFromArgList() with CallFirstTag() as first argument.

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

Can you give a concrete example of the resolution order (order of constexpr
"call" evaluations) for say

 {TaskPriority::USER_VISIBLE, MayBlock()};

?

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

Improved documentation.

+struct CallSecondTag {};
+struct CallFirstTag : CallSecondTag {};
+
+// Overload 1: Default value.
+template <class GetterType>
+constexpr auto GetTraitFromArgList(CallSecondTag, GetterType getter)
+    -> decltype(getter()) {
+  return getter();
+}
+
+// Overload 2: Discard first arg.
+template <class GetterType, class FirstArgType, class... ArgTypes>
+constexpr auto GetTraitFromArgList(CallSecondTag,
+                                   GetterType getter,
+                                   const FirstArgType&,
+                                   const ArgTypes&... args)
+    -> decltype(GetTraitFromArgList(CallFirstTag(), getter, args...)) {
+  return GetTraitFromArgList(CallFirstTag(), getter, args...);
+}
+
+// Overload 3: Get value of TaskTraits member from first arg.
+template <class GetterType,
+          class FirstArgType,
+          class... ArgTypes,
+          class Instantiation = decltype(
+              std::declval<GetterType>()(std::declval<FirstArgType>()))>
+constexpr typename GetterType::MemberType GetTraitFromArgList(
+    CallFirstTag,
+    GetterType getter,
+    const FirstArgType& first_arg,
+    const ArgTypes&...) {
+  return getter(first_arg);
+}
+
+template <typename ArgType>
+struct BooleanArgGetterType {
+  using MemberType = bool;
+  constexpr MemberType operator()(ArgType) const { return true; }
+  constexpr MemberType operator()() const { return false; }
+};
+
+template <typename ArgType, ArgType DefaultValue>
+struct EnumArgGetterType {
+  using MemberType = ArgType;
+  constexpr MemberType operator()(ArgType arg) const { return arg; }
+  constexpr MemberType operator()() const { return DefaultValue; }
+};
+
+}  // namespace internal
+
 // 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.
+  // 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.
   //
-  // 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).
-  //
-  // 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};
+  template <class... ArgTypes>
+  constexpr TaskTraits(const ArgTypes&... args)
+      : priority_(internal::GetTraitFromArgList(
+            internal::CallFirstTag(),
+            internal::EnumArgGetterType<base::TaskPriority,
+                                        base::TaskPriority::INHERITED>(),
+            args...)),
+        shutdown_behavior_(internal::GetTraitFromArgList(
+            internal::CallFirstTag(),
+            internal::EnumArgGetterType<
+                base::TaskShutdownBehavior,
+                base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN>(),
+            args...)),
+        may_block_(internal::GetTraitFromArgList(
+            internal::CallFirstTag(),
+            internal::BooleanArgGetterType<base::MayBlock>(),
+            args...)),
+        with_base_sync_primitives_(internal::GetTraitFromArgList(
+            internal::CallFirstTag(),
+            internal::BooleanArgGetterType<base::WithBaseSyncPrimitives>(),
+            args...)) {}
 
-  // Applies |priority| to tasks with these traits.
-  TaskTraits& WithPriority(TaskPriority priority);
+  constexpr TaskTraits(const TaskTraits& other) = default;
+  TaskTraits& operator=(const TaskTraits& other) = default;
 
-  // Applies |shutdown_behavior| to tasks with these traits.
+  // Deprecated.
+  TaskTraits& WithPriority(TaskPriority priority);
   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 +238,17 @@ 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_; }
+
+  // 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_;
   TaskShutdownBehavior shutdown_behavior_;
+  bool may_block_;
+  bool with_base_sync_primitives_;
 };
 
 // Returns string literals for the enums defined in this file. These methods