Add constexpr TaskTraits constructor.

base/task_scheduler/task_traits_details.h

+// Copyright 2017 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_DETAILS_H_
+#define BASE_TASK_SCHEDULER_TASK_TRAITS_DETAILS_H_
+
+#include <type_traits>
+#include <utility>
+
+namespace base {
+namespace internal {
+
+// HasArgOfType<CheckedType, ArgTypes...>::value is true iff a type in ArgTypes
+// matches CheckedType.
+template <class...>
+struct HasArgOfType : std::false_type {};
+template <class CheckedType, class FirstArgType, class... ArgTypes>
+struct HasArgOfType<CheckedType, FirstArgType, ArgTypes...>
+    : std::conditional<std::is_same<CheckedType, FirstArgType>::value,
+                       std::true_type,
+                       HasArgOfType<CheckedType, ArgTypes...>>::type {};
+
+// ArgsAreConvertible<CheckedType, ArgTypes...>::value is true iff all types in
+// ArgTypes are convertible to CheckedType.
+template <class...>
+struct ArgsAreConvertible : std::true_type {};
+template <class CheckedType, class FirstArgType, class... ArgTypes>
+struct ArgsAreConvertible<CheckedType, FirstArgType, ArgTypes...>
+    : std::conditional<std::is_convertible<FirstArgType, CheckedType>::value,
+                       ArgsAreConvertible<CheckedType, ArgTypes...>,
+                       std::false_type>::type {};
+
+// When the following call is made:
+//    GetValueFromArgListImpl(CallFirstTag(), GetterType(), args...);
+// The compiler prefers using the first overload of GetValueFromArgListImpl(),
+// because the type of the first argument matches exactly. This overload returns
+// getter.GetValueFromArg(first_arg), where |first_arg| is the first element in
+// |args|. If getter.GetValueFromArg(first_arg) isn't defined, the compiler uses
+// the second or the third overload instead (depending on whether |args| is
+// empty). The second overload discards |first_arg| and makes a recursive call
+// to GetValueFromArgListImpl() with CallFirstTag() as first argument. The third
+// overload returns getter.GetDefaultValue().
+
+// Tag dispatching.
+struct CallSecondTag {};
+struct CallFirstTag : CallSecondTag {};
+
+// Overload 1: Get value from first argument. Check that no argument in |args|
+// has the same type as |first_arg|.
+template <class GetterType,
+          class FirstArgType,
+          class... ArgTypes,
+          class TestGetValueFromArgDefined =
+              decltype(std::declval<GetterType>().GetValueFromArg(
+                  std::declval<FirstArgType>()))>
+constexpr typename GetterType::ValueType GetValueFromArgListImpl(
+    CallFirstTag,
+    GetterType getter,
+    const FirstArgType& first_arg,
+    const ArgTypes&... args) {
+  static_assert(!HasArgOfType<FirstArgType, ArgTypes...>::value,
+                "Multiple arguments of the same type were provided to the "
+                "constructor of TaskTraits.");
+  return getter.GetValueFromArg(first_arg);
+}
+
+// Overload 2: Discard first argument.
+template <class GetterType, class FirstArgType, class... ArgTypes>
+constexpr typename GetterType::ValueType GetValueFromArgListImpl(
+    CallSecondTag,
+    GetterType getter,
+    const FirstArgType&,
+    const ArgTypes&... args) {
+  return GetValueFromArgListImpl(CallFirstTag(), getter, args...);
+}
+
+// Overload 3: Default value.
+template <class GetterType>
+constexpr typename GetterType::ValueType GetValueFromArgListImpl(
+    CallSecondTag,

[gab, 2017/04/28 18:03:40]

I guess it doesn't matter that this is CallFirstTag or CallSecondTag, right?
It's the only signature that matches an empty list of |args|.

So I'd say make this the first overload with a CallFirstTag, then the more
advanced one the second overload with CallFirstTag, only the one dropping the
param needs a CallSecondTag? That makes it easier to understand the tags IMO.

i.e.:
1) Default when |args| is empty.
2) Get arg from first arg.
3) Skip first arg and go back to (1) if (2) failed.

[fdoray, 2017/04/28 18:40:36]

Done.

+    GetterType getter) {
+  return getter.GetDefaultValue();
+}
+
+// If there is an argument |arg_of_type| of type Getter::ArgType in |args|,
+// returns getter.GetValueFromArg(arg_of_type). If there are more than one
+// argument of type Getter::ArgType in |args|, generates a compile-time error.
+// Otherwise, returns getter.GetDefaultValue().
+//
+// |getter| must provide:
+//
+// ValueType:
+//     The return type of GetValueFromArgListImpl().
+//
+// ArgType:
+//     The type of the argument from which GetValueFromArgListImpl() derives its
+//     return value.
+//
+// ValueType GetValueFromArg(ArgType):
+//     Converts an argument of type ArgType into a value returned by
+//     GetValueFromArgListImpl().
+//
+// ValueType GetDefaultValue():
+//     Returns the value returned by GetValueFromArgListImpl() if none of its
+//     arguments is of type ArgType.
+template <class GetterType, class... ArgTypes>
+constexpr typename GetterType::ValueType GetValueFromArgList(
+    GetterType getter,
+    const ArgTypes&... args) {
+  return GetValueFromArgListImpl(CallFirstTag(), getter, args...);
+}
+
+template <typename ArgType>
+struct BooleanArgGetter {
+  using ValueType = bool;
+  constexpr ValueType GetValueFromArg(ArgType) const { return true; }
+  constexpr ValueType GetDefaultValue() const { return false; }
+};
+
+template <typename ArgType, ArgType DefaultValue>
+struct EnumArgGetter {
+  using ValueType = ArgType;
+  constexpr ValueType GetValueFromArg(ArgType arg) const { return arg; }
+  constexpr ValueType GetDefaultValue() const { return DefaultValue; }
+};
+
+}  // namespace internal
+}  // namespace base
+
+#endif  // BASE_TASK_SCHEDULER_TASK_TRAITS_DETAILS_H_