Neither libstdc++ 5.1.0 nor the trunk version implement LWG defect reports 2275 and 2301 that make std::tuple utilities std::forward_as_tuple and std::tie constexpr
Both of these have been implemented in libc++ for a while now:
Since both resolutions only require C++11 style constexpr support, they could also be backported to earlier versions in -std=c++1y mode.
Date: Sat May 2 11:20:03 2015
New Revision: 222719
* include/std/tuple (forward_as_tuple, tie): Add constexpr.
* testsuite/20_util/tuple/creation_functions/constexpr.cc: Uncomment
and fix tests for forward_as_tuple and tie.
Fixed on trunk so far.
Ping to get this merged into the upcoming 5.4.
This was fixed for GCC 6, but I'm not going to backport it.