Fix path::iterator post-increment and post-decrement

Backport from mainline
2017-10-19  Jonathan Wakely  <jwakely@redhat.com>

	* include/experimental/bits/fs_path.h (path::iterator++(int))
	(path::iterator--(int)): Fix for paths with only one component.
	* testsuite/experimental/filesystem/path/itr/traversal.cc: Test
	post-increment and post-decrement.

From-SVN: r253942

libstdc++-v3/ChangeLog

@@ -1,3 +1,13 @@
+2017-10-20  Jonathan Wakely  <jwakely@redhat.com>
+
+	Backport from mainline
+	2017-10-19  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/experimental/bits/fs_path.h (path::iterator++(int))
+	(path::iterator--(int)): Fix for paths with only one component.
+	* testsuite/experimental/filesystem/path/itr/traversal.cc: Test
+	post-increment and post-decrement.
+
 2017-09-21  Jonathan Wakely  <jwakely@redhat.com>
 
 	* testsuite/25_algorithms/clamp/1.cc: Fix order of arguments and

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

@@ -724,10 +724,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
     pointer   operator->() const { return std::__addressof(**this); }
 
     iterator& operator++();
-    iterator  operator++(int) { auto __tmp = *this; ++_M_cur; return __tmp; }
+    iterator  operator++(int) { auto __tmp = *this; ++*this; return __tmp; }
 
     iterator& operator--();
-    iterator  operator--(int) { auto __tmp = *this; --_M_cur; return __tmp; }
+    iterator  operator--(int) { auto __tmp = *this; --*this; return __tmp; }
 
     friend bool operator==(const iterator& __lhs, const iterator& __rhs)
     { return __lhs._M_equals(__rhs); }

libstdc++-v3/testsuite/experimental/filesystem/path/itr/traversal.cc

@@ -79,9 +79,28 @@ test02()
   }
 }
 
+void
+test03()
+{
+  path paths[] = { "single", "multiple/elements" };
+  for (const path& p : paths)
+    for (auto iter = p.begin(); iter != p.end(); ++iter)
+    {
+      auto iter2 = iter;
+      ++iter;
+      iter2++;
+      VERIFY( iter2 == iter );
+      auto iter3 = iter;
+      --iter3;
+      iter2--;
+      VERIFY( iter2 == iter3 );
+    }
+}
+
 int
 main()
 {
   test01();
   test02();
+  test03();
 }