PR libstdc++/82917 add missing returns in <fstream>

	PR libstdc++/82917
	* include/std/fstream (basic_ifstream::open, basic_ofstream::open)
	(basic_fstream::open): Fix missing return.

From-SVN: r254638

libstdc++-v3/ChangeLog

+2017-11-10  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/82917
+	* include/std/fstream (basic_ifstream::open, basic_ofstream::open)
+	(basic_fstream::open): Fix missing return.
+
 2017-11-07  Jonathan Wakely  <jwakely@redhat.com>
 
 	* src/filesystem/ops-common.h (make_file_type) [S_ISSOCK]: Only use

libstdc++-v3/include/std/fstream

@@ -663,7 +663,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       template<typename _Path>
 	auto
 	open(const _Path& __s, ios_base::openmode __mode = ios_base::in)
-	-> decltype(_M_filebuf.open(__s, __mode))
+	-> decltype((void)_M_filebuf.open(__s, __mode))
 	{ open(__s.c_str(), __mode); }
 #endif // C++17
 #endif // C++11
@@ -891,7 +891,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       template<typename _Path>
 	auto
 	open(const _Path& __s, ios_base::openmode __mode = ios_base::out)
-	-> decltype(_M_filebuf.open(__s, __mode))
+	-> decltype((void)_M_filebuf.open(__s, __mode))
 	{ open(__s.c_str(), __mode); }
 #endif // C++17
 #endif // C++11
@@ -1118,7 +1118,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	auto
 	open(const _Path& __s,
 	     ios_base::openmode __mode = ios_base::in | ios_base::out)
-	-> decltype(_M_filebuf.open(__s, __mode))
+	-> decltype((void)_M_filebuf.open(__s, __mode))
 	{ open(__s.c_str(), __mode); }
 #endif // C++17
 #endif // C++11