operator-(const time_point &, const duration &) mishandles unsigned numbers

[jlanca]

The binary subtraction operation tp - d is implemented as tp + (-d). Because the unary minus operator is used, the result is wrong if the underlying representation type is unsigned.

For instance :

#include <cassert>
#include <chrono>
#include <iostream>
#include <boost/chrono.hpp>
 
// namespace chrono = std::chrono;
// using std::micro;
namespace chrono = boost::chrono;
using boost::micro;
 
int main() {
 
  const auto tp = chrono::system_clock::time_point(chrono::milliseconds(0));
  chrono::duration<uint32_t, micro> delay(1);
  std::cout << (tp - delay).time_since_epoch().count() << std::endl;
  assert((tp - delay).time_since_epoch().count() == -1000);
  return 0;
}

Running this program produces the following output:

4294967295000
a: /tmp/a.cpp:16: int main(): Assertion `(tp - delay).time_since_epoch().count() == -1000' failed.

Note that the std::chrono from gcc does not exhibit this defect.