PR libstdc++/81468 fix test for duration conversions

	PR libstdc++/81468
	* testsuite/20_util/duration/cons/dr1177.cc: Fix incorrect test and
	improve static assertion messages.

From-SVN: r252950

libstdc++-v3/ChangeLog

 2017-09-18  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/81468
+	* testsuite/20_util/duration/cons/dr1177.cc: Fix incorrect test and
+	improve static assertion messages.
+
 	* include/std/utility (_Itup_cat, _Make_integer_sequence): Remove.
 	(_Build_index_tuple, make_integer_sequence): Use built-in to generate
 	pack expansion.

libstdc++-v3/testsuite/20_util/duration/cons/dr1177.cc

@@ -36,6 +36,6 @@ static_assert(is_constructible<duration<int>, duration<long>>{},
 static_assert(!is_constructible<duration<int>, duration<int, ratio<2,3>>>{},
     "cannot convert duration to one with different period");
 static_assert(is_constructible<duration<float>, duration<int, ratio<2,3>>>{},
-    "unless it has a floating-point representation");
-static_assert(is_constructible<duration<float>, duration<int, ratio<1,3>>>{},
-    "or a period that is an integral multiple of the original");
+    "... unless the result type has a floating-point representation");
+static_assert(is_constructible<duration<int, ratio<1,3>>, duration<int>>{},
+    "... or the original's period is a multiple of the result's period");