Mismatch in return type of converting function is now hard error.

Change the implementation, in the way that mismatch in return
type of the from/to_sys/utc functions (not returning time_point,
or returning time_point with inappropriate clock) leads to
hard error.

Added appropariate fail test for to_sys function, including:
* returning an int
* returning time_point of wrong clock
* reutrning reference to time_point

test/tz_test/to_sys_return_int.fail.cpp

@@ -0,0 +1,49 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2017 Tomasz Kamiński
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#include "tz.h"
+
+struct bad_clock 
+{
+  using duration = std::chrono::system_clock::duration;
+  using rep = duration::rep;
+  using period = duration::period;
+  using time_point = std::chrono::time_point<bad_clock, duration>;
+
+  template<typename Duration>
+  static
+  int 
+  to_sys(std::chrono::time_point<bad_clock, Duration> const& tp)
+  {
+    return tp.time_since_epoch().count();
+  }
+};
+
+int
+main()
+{
+    using namespace date;
+    using sys_clock = std::chrono::system_clock;
+
+    auto bt = bad_clock::time_point();
+    clock_cast<sys_clock>(bt);
+}

test/tz_test/to_sys_return_reference.cpp

@@ -0,0 +1,51 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2017 Tomasz Kamiński
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#include "tz.h"
+
+struct bad_clock 
+{
+  using duration = std::chrono::system_clock::duration;
+  using rep = duration::rep;
+  using period = duration::period;
+  using time_point = std::chrono::time_point<bad_clock, duration>;
+
+  template<typename Duration>
+  static
+  date::sys_time<Duration> const&
+  to_sys(std::chrono::time_point<bad_clock, Duration> const& tp)
+  {
+    static date::sys_time<Duration> val;
+    val = date::sys_time<Duration>(tp.time_since_epoch());
+    return val;
+  }
+};
+
+int
+main()
+{
+    using namespace date;
+    using sys_clock = std::chrono::system_clock;
+
+    auto bt = bad_clock::time_point();
+    clock_cast<sys_clock>(bt);
+}

test/tz_test/to_sys_return_utc_time.fail.cpp

@@ -0,0 +1,49 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2017 Tomasz Kamiński
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#include "tz.h"
+
+struct bad_clock 
+{
+  using duration = std::chrono::system_clock::duration;
+  using rep = duration::rep;
+  using period = duration::period;
+  using time_point = std::chrono::time_point<bad_clock, duration>;
+
+  template<typename Duration>
+  static
+  date::utc_time<Duration> 
+  to_sys(std::chrono::time_point<bad_clock, Duration> const& tp)
+  {
+    return utc_time<Duration>(tp.time_since_epoch());
+  }
+};
+
+int
+main()
+{
+    using namespace date;
+    using sys_clock = std::chrono::system_clock;
+
+    auto bt = bad_clock::time_point();
+    clock_cast<sys_clock>(bt);
+}