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.

3 files changed, 30 insertions, 1 deletions

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 49f2b59e3ae..0df7126f7c9 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,9 @@
+2020-02-27  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/bits/boost_concept_check.h (__function_requires): Add
+	_GLIBCXX14_CONSTEXPR.
+	* testsuite/25_algorithms/min/concept_checks.cc: New test.
+
 2020-02-26  Patrick Palka  <ppalka@redhat.com>
 
 	PR libstdc++/93936
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);