Implement the default_searcher part of C++20 p1032 Misc constexpr bits.

2019-11-15  Edward Smith-Rowland  <3dw4rd@verizon.net>

	Implement the default_searcher part of C++20 p1032 Misc constexpr bits.
	* include/std/functional
	(default_searcher, default_searcher::operator()): Constexpr.
	* testsuite/20_util/function_objects/constexpr_searcher.cc: New.

From-SVN: r278317

libstdc++-v3/ChangeLog

+2019-11-15  Edward Smith-Rowland  <3dw4rd@verizon.net>
+
+	Implement the default_searcher part of C++20 p1032 Misc constexpr bits.
+	* include/std/functional
+	(default_searcher, default_searcher::operator()): Constexpr.
+	* testsuite/20_util/function_objects/constexpr_searcher.cc: New.
+
 2019-11-15  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/bits/iterator_concepts.h (weakly_incrementable): Adjust.

libstdc++-v3/include/std/functional

@@ -1000,6 +1000,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     class default_searcher
     {
     public:
+      _GLIBCXX20_CONSTEXPR
       default_searcher(_ForwardIterator1 __pat_first,
 		       _ForwardIterator1 __pat_last,
 		       _BinaryPredicate __pred = _BinaryPredicate())
@@ -1007,7 +1008,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { }
 
       template<typename _ForwardIterator2>
-        pair<_ForwardIterator2, _ForwardIterator2>
+	_GLIBCXX20_CONSTEXPR
+	pair<_ForwardIterator2, _ForwardIterator2>
 	operator()(_ForwardIterator2 __first, _ForwardIterator2 __last) const
 	{
 	  _ForwardIterator2 __first_ret =

libstdc++-v3/testsuite/20_util/function_objects/constexpr_searcher.cc

+// Copyright (C) 2014-2019 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 <functional>
+#include <string_view>
+
+const std::string_view
+patt = "World";
+
+constexpr std::string_view
+greet = "Hello, Humongous World of Wonder!!!";
+
+const std::wstring_view
+wpatt = L"World";
+
+constexpr std::wstring_view
+wgreet = L"Hello, Humongous World of Wonder!!!";
+
+constexpr bool
+test_searcher()
+{
+  auto ok = true;
+
+  const std::default_searcher search(patt.begin(), patt.end(),
+				     std::equal_to<>());
+  const auto find = search(greet.begin(), greet.end());
+
+  const std::default_searcher wsearch(wpatt.begin(), wpatt.end(),
+				      std::equal_to<>());
+  const auto wfind = wsearch(wgreet.begin(), wgreet.end());
+
+  return ok;
+}
+
+static_assert(test_searcher());