Add [[nodiscard]] attribute to C++17 components

	* include/bits/fs_path.h (path::empty): Add nodiscard attribute.
	* include/bits/range_access.h (empty): Likewise.
	* include/std/string_view (basic_string_view::empty): Likewise.
	* testsuite/21_strings/basic_string_view/capacity/empty_neg.cc: New
	test.
	* testsuite/24_iterators/range_access_cpp17_neg.cc: New test.
	* testsuite/27_io/filesystem/path/query/empty_neg.cc: New test.

From-SVN: r255124

libstdc++-v3/ChangeLog

 2017-11-23  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/fs_path.h (path::empty): Add nodiscard attribute.
+	* include/bits/range_access.h (empty): Likewise.
+	* include/std/string_view (basic_string_view::empty): Likewise.
+	* testsuite/21_strings/basic_string_view/capacity/empty_neg.cc: New
+	test.
+	* testsuite/24_iterators/range_access_cpp17_neg.cc: New test.
+	* testsuite/27_io/filesystem/path/query/empty_neg.cc: New test.
+
 	PR libstdc++/83134
 	* include/std/type_traits (__not_): Explicitly convert to bool.
 	* testsuite/20_util/declval/requirements/1_neg.cc: Adjust dg-error.

libstdc++-v3/include/bits/fs_path.h

@@ -370,7 +370,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
     // query
 
-    bool empty() const noexcept { return _M_pathname.empty(); }
+    [[nodiscard]] bool empty() const noexcept { return _M_pathname.empty(); }
     bool has_root_name() const;
     bool has_root_directory() const;
     bool has_root_path() const;

libstdc++-v3/include/bits/node_handle.h

@@ -62,7 +62,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       explicit operator bool() const noexcept { return _M_ptr != nullptr; }
 
-      bool empty() const noexcept { return _M_ptr == nullptr; }
+      [[nodiscard]] bool empty() const noexcept { return _M_ptr == nullptr; }
 
     protected:
       constexpr _Node_handle_common() noexcept : _M_ptr(), _M_alloc() {}

libstdc++-v3/include/bits/range_access.h

@@ -257,7 +257,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __cont  Container.
    */
   template <typename _Container>
-    constexpr auto
+    [[nodiscard]] constexpr auto
     empty(const _Container& __cont) noexcept(noexcept(__cont.empty()))
     -> decltype(__cont.empty())
     { return __cont.empty(); }
@@ -267,7 +267,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __array  Container.
    */
   template <typename _Tp, size_t _Nm>
-    constexpr bool
+    [[nodiscard]] constexpr bool
     empty(const _Tp (&/*__array*/)[_Nm]) noexcept
     { return false; }
 
@@ -276,7 +276,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __il  Initializer list.
    */
   template <typename _Tp>
-    constexpr bool
+    [[nodiscard]] constexpr bool
     empty(initializer_list<_Tp> __il) noexcept
     { return __il.size() == 0;}
 

libstdc++-v3/include/std/string_view

@@ -160,7 +160,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		/ sizeof(value_type) / 4;
       }
 
-      constexpr bool
+      [[nodiscard]] constexpr bool
       empty() const noexcept
       { return this->_M_len == 0; }
 

libstdc++-v3/testsuite/21_strings/basic_string_view/capacity/empty_neg.cc

+// Copyright (C) 2017 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++17" }
+// { dg-do compile { target c++17 } }
+
+#include <string_view>
+
+void
+test01()
+{
+  std::string_view s;
+  s.empty();  // { dg-warning "ignoring return value" }
+}

libstdc++-v3/testsuite/24_iterators/range_access_cpp17_neg.cc

+// Copyright (C) 2017 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++17" }
+// { dg-do compile { target c++17 } }
+
+#include <iterator>
+#include <initializer_list>
+
+void
+test01()
+{
+  struct A { bool empty() const { return true; } };
+  A a;
+  std::empty(a);  // { dg-warning "ignoring return value" }
+}
+
+void
+test02()
+{
+  int a[2];
+  std::empty(a);  // { dg-warning "ignoring return value" }
+}
+
+void
+test03()
+{
+  std::initializer_list<int> a{};
+  std::empty(a);  // { dg-warning "ignoring return value" }
+}

libstdc++-v3/testsuite/27_io/filesystem/path/query/empty_neg.cc

+// Copyright (C) 2017 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++17" }
+// { dg-do compile { target c++17 } }
+
+#include <filesystem>
+
+void
+test01()
+{
+  std::filesystem::path p;
+  p.empty();  // { dg-warning "ignoring return value" }
+}