Fix concatenation bug in filesystem::path

When erasing a trailing empty filename component, the output iterator
was not decremented, causing the next component to be created at the
wrong position.

	* src/filesystem/std-path.cc (path::operator+=(const path&)): Fix
	incorrect treatment of empty filename after trailing slash.
	* testsuite/27_io/filesystem/path/concat/path.cc: Test problem case.

From-SVN: r267574

libstdc++-v3/ChangeLog

 2019-01-04  Jonathan Wakely  <jwakely@redhat.com>
 
+	* src/filesystem/std-path.cc (path::operator+=(const path&)): Fix
+	incorrect treatment of empty filename after trailing slash.
+	* testsuite/27_io/filesystem/path/concat/path.cc: Test problem case.
+
 	* testsuite/21_strings/basic_string/modifiers/assign/char/
 	move_assign_optim.cc: Avoid spurious failure when -fno-inline added
 	to test flags.

libstdc++-v3/src/filesystem/std-path.cc

@@ -945,7 +945,7 @@ path::operator+=(const path& p)
       else if (orig_filenamelen == 0 && it != last)
 	{
 	  // Remove empty filename at end of original path.
-	  _M_cmpts.erase(std::prev(output));
+	  _M_cmpts.erase(--output);
 	}
 
       if (it != last && it->_M_type() == _Type::_Root_name)

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

@@ -59,9 +59,18 @@ test02()
   }
 }
 
+void
+test03()
+{
+  path p = "a/";
+  p += path("/b");
+  compare_paths(p, "a//b");
+}
+
 int
 main()
 {
   test01();
   test02();
+  test03();
 }