Fix std::midpoint(T*, T*) for reversed arguments

	* include/std/numeric (midpoint(T*, T*)): Fix incorrect result.
	* testsuite/26_numerics/midpoint/pointer.cc: Change "compile" test
	to "run".

From-SVN: r271606

libstdc++-v3/ChangeLog

 2019-05-24  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/numeric (midpoint(T*, T*)): Fix incorrect result.
+	* testsuite/26_numerics/midpoint/pointer.cc: Change "compile" test
+	to "run".
+
 	* testsuite/20_util/shared_ptr/cons/alias-rval.cc: Fix test.
 	* testsuite/20_util/shared_ptr/cons/alias.cc: Remove unused function.
 

libstdc++-v3/include/std/numeric

@@ -195,7 +195,7 @@ template<typename _Tp>
     enable_if_t<__and_v<is_object<_Tp>, bool_constant<sizeof(_Tp) != 0>>, _Tp*>
     midpoint(_Tp* __a, _Tp* __b) noexcept
     {
-      return __a > __b ? __b + (__a - __b) / 2 : __a + (__b - __a) / 2;
+      return __a  + (__b - __a) / 2;
     }
 #endif // C++20
 

libstdc++-v3/testsuite/26_numerics/midpoint/pointer.cc

@@ -16,7 +16,7 @@
 // <http://www.gnu.org/licenses/>.
 
 // { dg-options "-std=gnu++2a" }
-// { dg-do compile { target c++2a } }
+// { dg-do run { target c++2a } }
 
 #include <numeric>
 #include <climits>