libstdc++: midpoint should not constrain T is complete (LWG 3200)

	* include/std/numeric (midpoint(T8, T*)): Do not check for complete
	type during overload resolution, use static assert instead (LWG 3200).
	* testsuite/26_numerics/midpoint/pointer.cc: Do not test with
	incomplete type.
	* testsuite/26_numerics/midpoint/pointer_neg.cc: New test.

libstdc++-v3/ChangeLog

 2020-02-19  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/numeric (midpoint(T8, T*)): Do not check for complete
+	type during overload resolution, use static assert instead (LWG 3200).
+	* testsuite/26_numerics/midpoint/pointer.cc: Do not test with
+	incomplete type.
+	* testsuite/26_numerics/midpoint/pointer_neg.cc: New test.
+
 	* include/std/span (span(T (&)[N])): Use non-deduced context to
 	prevent first parameter from interfering with class template argument
 	deduction (LWG 3369).

libstdc++-v3/include/std/numeric

@@ -208,10 +208,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _Tp>
-    constexpr
-    enable_if_t<__and_v<is_object<_Tp>, bool_constant<sizeof(_Tp) != 0>>, _Tp*>
+    constexpr enable_if_t<is_object_v<_Tp>, _Tp*>
     midpoint(_Tp* __a, _Tp* __b) noexcept
     {
+      static_assert( sizeof(_Tp) != 0, "type must be complete" );
       return __a  + (__b - __a) / 2;
     }
 _GLIBCXX_END_NAMESPACE_VERSION

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

@@ -19,7 +19,6 @@
 // { dg-do run { target c++2a } }
 
 #include <numeric>
-#include <climits>
 #include <testsuite_hooks.h>
 
 const int* p = nullptr;
@@ -36,7 +35,6 @@ template<typename T> constexpr bool no_midpoint()
 static_assert(no_midpoint<void>());
 static_assert(no_midpoint<int()>());
 static_assert(no_midpoint<int&>());
-static_assert(no_midpoint<struct Incomplete>());
 
 constexpr int ca[3] = {};
 static_assert( std::midpoint(ca, ca+3) == ca+1 );

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

+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <numeric>
+
+// LWG 3200. midpoint should not constrain T is complete
+struct Incomplete* pinc = 0;
+auto p1 = std::midpoint(pinc, pinc); // { dg-error "here" }
+
+// { dg-error "incomplete type" "" { target *-*-* } 0 }