Fix filesystem::last_write_time failure with 32-bit time_t

	* testsuite/27_io/filesystem/operations/last_write_time.cc: Fix
	test failures on targets with 32-bit time_t.

From-SVN: r267811

libstdc++-v3/ChangeLog

 2019-01-10  Jonathan Wakely  <jwakely@redhat.com>
 
+	* testsuite/27_io/filesystem/operations/last_write_time.cc: Fix
+	test failures on targets with 32-bit time_t.
+
 	* include/bits/erase_if.h: Define __cpp_lib_erase_if.
 	* include/std/deque: Likewise.
 	* include/std/forward_list: Likewise.

libstdc++-v3/testsuite/27_io/filesystem/operations/last_write_time.cc

@@ -22,6 +22,7 @@
 // 15.25 Permissions [fs.op.last_write_time]
 
 #include <filesystem>
+#include <limits>
 #include <testsuite_fs.h>
 #include <testsuite_hooks.h>
 
@@ -141,14 +142,27 @@ test02()
   VERIFY( !ec );
   VERIFY( approx_equal(last_write_time(f.path), time) );
 
+  if (std::numeric_limits<std::time_t>::max()
+      < std::numeric_limits<std::int64_t>::max())
+    return; // file clock's epoch is out of range for 32-bit time_t
+
   ec = bad_ec;
+  // The file clock's epoch:
   time = time_type();
   last_write_time(f.path, time, ec);
   VERIFY( !ec );
   VERIFY( approx_equal(last_write_time(f.path), time) );
 
   ec = bad_ec;
-  time -= std::chrono::milliseconds(1000 * 60 * 10 + 15);
+  // A time after the epoch
+  time += std::chrono::milliseconds(1000 * 60 * 10 + 15);
+  last_write_time(f.path, time, ec);
+  VERIFY( !ec );
+  VERIFY( approx_equal(last_write_time(f.path), time) );
+
+  ec = bad_ec;
+  // A time before than the epoch
+  time -= std::chrono::milliseconds(1000 * 60 * 20 + 15);
   last_write_time(f.path, time, ec);
   VERIFY( !ec );
   VERIFY( approx_equal(last_write_time(f.path), time) );