Add explicit TimeDelta constexpr copy constructor.

base/time/time_win_unittest.cc

 // Copyright (c) 2012 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.
 
 #include <windows.h>
 #include <mmsystem.h>
 #include <process.h>
 #include <stdint.h>
 
 #include <cmath>
 #include <limits>
 #include <vector>
 
 #include "base/threading/platform_thread.h"
 #include "base/time/time.h"
 #include "base/win/registry.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace base {
 namespace {
 
+// For TimeDelta::ConstexprInitialization
+constexpr int kExpectedDeltaInMilliseconds = 10;
+constexpr TimeDelta kConstexprTimeDelta =
+    TimeDelta::FromMilliseconds(kExpectedDeltaInMilliseconds);
+
 class MockTimeTicks : public TimeTicks {
  public:
   static DWORD Ticker() {
     return static_cast<int>(InterlockedIncrement(&ticker_));
   }
 
   static void InstallTicker() {
     old_tick_function_ = SetMockTickFunction(&Ticker);
     ticker_ = -5;
   }
@@ skipping 251 matching lines @@
     const double converted_microseconds_since_origin =
         static_cast<double>((converted_value - TimeTicks()).InMicroseconds());
     EXPECT_NEAR(expected_microseconds_since_origin,
                 converted_microseconds_since_origin,
                 1.0)
         << "ticks=" << ticks << ", to be converted via logic path: "
         << (ticks < Time::kQPCOverflowThreshold ? "FAST" : "SAFE");
   }
 }
 
+TEST(TimeDelta, ConstexprInitialization) {
+  // Make sure that TimeDelta works around crbug.com/635974
+  EXPECT_EQ(kExpectedDeltaInMilliseconds, kConstexprTimeDelta.InMilliseconds());

[Jeffrey Yasskin, 2016/08/12 23:07:44]

You're testing a bug in constexpr variables, so you shouldn't really use
constexpr variables on both sides of the EXPECT_EQ: the bug could cancel itself
out.

I think that's unlikely in this case because of the nature of the bug, but it'd
be a better test with a literal '10' there.

+}
+
 }  // namespace base