let coarse clocks use underlying clock durations

Summary: Rather than having coarser `duration` types, the only purpose of which is signalling coarseness via a hijacked channel.

Reviewed By: simpkins

Differential Revision: D29221055

fbshipit-source-id: 025a8b94972ee77a7d4a5bd34f269c492aaf2c4e

folly/Chrono.h

@@ -176,7 +176,7 @@ namespace folly {
 namespace chrono {
 
 struct coarse_steady_clock {
-  using duration = std::chrono::milliseconds;
+  using duration = std::chrono::steady_clock::duration;
   using rep = duration::rep;
   using period = duration::period;
   using time_point = std::chrono::time_point<coarse_steady_clock>;
@@ -200,7 +200,7 @@ struct coarse_steady_clock {
 };
 
 struct coarse_system_clock {
-  using duration = std::chrono::milliseconds;
+  using duration = std::chrono::system_clock::duration;
   using rep = duration::rep;
   using period = duration::period;
   using time_point = std::chrono::time_point<coarse_system_clock>;

folly/test/ChronoTest.cpp

@@ -21,6 +21,16 @@
 using namespace std::chrono;
 using namespace folly::chrono;
 
+static_assert( //
+    std::is_same_v<
+        steady_clock::time_point::duration,
+        coarse_steady_clock::time_point::duration>);
+
+static_assert( //
+    std::is_same_v<
+        system_clock::time_point::duration,
+        coarse_system_clock::time_point::duration>);
+
 namespace {
 
 class ChronoTest : public testing::Test {};