[PATCH] libstdc++: Make _GLIBCXX_CONCEPT_CHECKS more constexpr-friendly

[Jonathan Wakely]

Although most of the old-style "concept checks" are only really usable
with C++98 because they enforce the wrong things, this is a simple
change that makes them a bit more useful for C++14 and up.

        * include/bits/boost_concept_check.h (__function_requires): Add
        _GLIBCXX14_CONSTEXPR.
        * testsuite/25_algorithms/min/concept_checks.cc: New test.

Tested powerpc64le-linux. These checks aren't enabled by default so
this is safe to change for stage4. Committed to master.


commit eb8e6a30a442c4c12dc903d6e1817b223bbed4a3
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Thu Feb 27 10:52:28 2020 +0000

    libstdc++: Make _GLIBCXX_CONCEPT_CHECKS more constexpr-friendly
    
    Although most of the old-style "concept checks" are only really usable
    with C++98 because they enforce the wrong things, this is a simple
    change that makes them a bit more useful for C++14 and up.
    
            * include/bits/boost_concept_check.h (__function_requires): Add
            _GLIBCXX14_CONSTEXPR.
            * testsuite/25_algorithms/min/concept_checks.cc: New test.

diff --git a/libstdc++-v3/include/bits/boost_concept_check.h 
b/libstdc++-v3/include/bits/boost_concept_check.h
index a555b3be568..f12c1bdc213 100644
--- a/libstdc++-v3/include/bits/boost_concept_check.h
+++ b/libstdc++-v3/include/bits/boost_concept_check.h
@@ -57,7 +57,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 // as possible at runtime, use as few resources as possible, and hopefully
 // be elided out of existence... hmmm.
 template <class _Concept>
-inline void __function_requires()
+_GLIBCXX14_CONSTEXPR inline void __function_requires()
 {
   void (_Concept::*__x)() _IsUnused = &_Concept::__constraints;
 }
diff --git a/libstdc++-v3/testsuite/25_algorithms/min/concept_checks.cc 
b/libstdc++-v3/testsuite/25_algorithms/min/concept_checks.cc
new file mode 100644
index 00000000000..ba61ceff340
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/min/concept_checks.cc
@@ -0,0 +1,23 @@
+// 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-do compile { target c++14 } }
+
+#define _GLIBCXX_CONCEPT_CHECKS 1
+#include <algorithm>
+
+constexpr int i = std::min(1, 2);