More of P0600 - '[[nodiscard]] in the Library' mark empty() as nodiscard in filesystem::path

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@318378 91177308-0d34-0410-b5e6-96231b3b80d8

include/experimental/filesystem

@@ -972,7 +972,8 @@ public:
     _LIBCPP_INLINE_VISIBILITY path extension()      const { return string_type(__extension()); }
 
     // query
-    _LIBCPP_INLINE_VISIBILITY bool empty() const _NOEXCEPT { return __pn_.empty(); }
+    _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY 
+    bool empty() const _NOEXCEPT { return __pn_.empty(); }
 
     _LIBCPP_INLINE_VISIBILITY bool has_root_name()      const { return !__root_name().empty(); }
     _LIBCPP_INLINE_VISIBILITY bool has_root_directory() const { return !__root_directory().empty(); }

test/std/experimental/filesystem/class.path/path.member/path.decompose/empty.fail.cpp

@@ -0,0 +1,28 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <experimental/filesystem>
+
+// class path
+
+// bool empty() const noexcept;
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
+// UNSUPPORTED: clang-3.3, clang-3.4, clang-3.5, clang-3.6, clang-3.7, clang-3.8
+
+#include <experimental/filesystem>
+
+#include "test_macros.h"
+
+int main ()
+{
+    std::experimental::filesystem::path c;
+    c.empty();  // expected-error {{ignoring return value of function declared with 'nodiscard' attribute}}
+}

test/std/experimental/filesystem/class.path/path.member/path.decompose/path.decompose.pass.cpp

@@ -177,6 +177,7 @@ void decompFilenameTest()
   for (auto const & TC : FilenameTestCases) {
     path p(TC.raw);
     assert(p == TC.raw);
+    ASSERT_NOEXCEPT(p.empty());
     
     assert(p.filename() == TC.filename);
     assert(p.has_filename() != TC.filename.empty());