Handling of temporaries in CLEF

[Thoemi09]

Description

When CLEF evaluates an expression, it tries to avoid unnecessary copies by moving objects whenever possible. This can lead to problems, when temporary objects are assigned to placeholders.

Steps to Reproduce

using namespace nda::clef::literals;
auto ex = i_ + i_;
nda::vector<int> vec = clef::eval(ex, i_ = nda::vector<int>{1, 2});
std::cout << vec << std::endl;

Expected behavior:

[2,4]

Actual behavior:

Segmentation fault: 11

The same problem arises, when we do partial evaluations:

using namespace nda::clef::literals;
auto ex = i_[0] + j_;
auto ex2 = clef::eval(ex, i_ = std::vector{1, 2, 3});
std::cout << clef::eval(ex2, j_ = 1) << std::endl;

Expected behavior:

2

Actual behavior:

1