basic_string.h (basic_string::basic_string()): Make noexcept conditional on allocator (LWG 2455).

	* include/bits/basic_string.h (basic_string::basic_string()): Make
	noexcept conditional on allocator (LWG 2455).

From-SVN: r223160

libstdc++-v3/ChangeLog

 2015-05-13  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/basic_string.h (basic_string::basic_string()): Make
+	noexcept conditional on allocator (LWG 2455).
+
 	* include/std/complex (polar): Check for negative rho (LWG 2459).
 
 	* include/experimental/tuple (apply): Handle pointers to member (LWG

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

@@ -377,7 +377,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       /**
        *  @brief  Default constructor creates an empty string.
        */
-      basic_string() _GLIBCXX_NOEXCEPT
+      basic_string()
+#if __cplusplus >= 201103L
+      noexcept(is_nothrow_default_constructible<_Alloc>::value)
+#endif
       : _M_dataplus(_M_local_data())
       { _M_set_length(0); }