mark clocks with spec tag

Summary:
So that if `std::chrono::steady_clock` and `folly::chrono::coarse_steady_clock` have the same spec, time points from the two clocks are compatible.

The intention is to provide a generic interface to determine whether it is safe to interpret a time-point from one clock as a time-point from another clock.

For example, it is safe to interpret a time-point from the coarse-steady-clock as a time-point from the steady-clock, and the code marks this by giving both clocks the same spec tag, but it is not safe to interpret a time-point from the system-clock as a time-point from the steady-clock, and the code marks this by giving the two clocks different or void spec tags.

Concrete code can already do this at will. The idea is to have a generic way to do this, from within an algorithm templated over all the clock types and having no special knowledge of any clock type.

Reviewed By: simpkins

Differential Revision: D29222091

fbshipit-source-id: b9442acd08716a46e704b6d800cc1cd1d0583e25

folly/Chrono.h

@@ -175,7 +175,43 @@ constexpr std::chrono::time_point<Clock, To> round(
 namespace folly {
 namespace chrono {
 
+//  steady_clock_spec
+//
+//  All clocks with this spec share epoch and tick rate.
+struct steady_clock_spec {};
+
+//  system_clock_spec
+//
+//  All clocks with this spec share epoch and tick rate.
+struct system_clock_spec {};
+
+//  clock_traits
+//
+//  Detects and reexports per-clock traits.
+//
+//  Specializable for clocks for which trait detection fails..
+template <typename Clock>
+struct clock_traits {
+ private:
+  template <typename C>
+  using detect_spec_ = typename C::folly_spec;
+
+ public:
+  using spec = detected_or_t<void, detect_spec_, Clock>;
+};
+
+template <>
+struct clock_traits<std::chrono::steady_clock> {
+  using spec = steady_clock_spec;
+};
+template <>
+struct clock_traits<std::chrono::system_clock> {
+  using spec = system_clock_spec;
+};
+
 struct coarse_steady_clock {
+  using folly_spec = steady_clock_spec;
+
   using duration = std::chrono::steady_clock::duration;
   using rep = duration::rep;
   using period = duration::period;
@@ -200,6 +236,8 @@ struct coarse_steady_clock {
 };
 
 struct coarse_system_clock {
+  using folly_spec = system_clock_spec;
+
   using duration = std::chrono::system_clock::duration;
   using rep = duration::rep;
   using period = duration::period;

folly/test/ChronoTest.cpp

@@ -21,11 +21,19 @@
 using namespace std::chrono;
 using namespace folly::chrono;
 
+static_assert( //
+    std::is_same_v<
+        clock_traits<steady_clock>::spec,
+        clock_traits<coarse_steady_clock>::spec>);
 static_assert( //
     std::is_same_v<
         steady_clock::time_point::duration,
         coarse_steady_clock::time_point::duration>);
 
+static_assert( //
+    std::is_same_v<
+        clock_traits<system_clock>::spec,
+        clock_traits<coarse_system_clock>::spec>);
 static_assert( //
     std::is_same_v<
         system_clock::time_point::duration,