Revert a change that breaks compilation.

[bangerth]

This reverts a part of #15422 (namely, https://github.com/dealii/dealii/pull/15422/files#diff-2c3b4f7640b5ae1a3b05deecf96447b2e09a22e8368ac24271b692d7eb6b29d1) and is necessary for me to build the library on my laptop.

@masterleinad Do your recall why you needed to make that change?

[tamiko]

In reference to #15383, and slightly faster than #15459

[tamiko]

I think clang-16 and gcc-13 have a different understanding of this corner case and a std::move is no longer necessary. Current master compiles for me with gcc-13 and clang-16 but older versions do fail.

[bangerth]

"Slightly faster"? I beat you by a full 20 minutes! ;-)

[masterleinad]

@masterleinad Do your recall why you needed to make that change?

clang-tidy was reporting that we return by const reference and that std::move would not have any effect.

[bangerth]

I see. I have to admit that I don't understand the warning then. The code in question is

      SE::PiecewiseVec piecewise_function;
      [...]
      expression = SE::piecewise(std::move(piecewise_function));

and we're calling the following function in SymEngine:

inline RCP<const Basic> piecewise(PiecewiseVec &&vec)
{
    return make_rcp<Piecewise>(std::move(vec));
}

Since piecewise() takes an rvalue reference, the std::move does make sense.

But perhaps I'm missing something. If you happen to have the warning message somewhere (or get it again, now that we have the original code back), feel free to open an issue!

[bangerth]

(It is of course also possible that we're using different versions of SymEngine and that that causes the difference. For reference, the function piecewise() in question is in symengine-0.8.1/include/symengine/logic.h, line 101, in my installation.)

[tamiko]

@bangerth The clang-16 and gcc-13 variants that accepted the removal of the std::move have symengine 0.10 installed, the once that failed (and hence the revert) are sitting on symengine 0.90 or older.

[bangerth]

So the newer variants are ok with both forms? In that case, we should leave things as they currently (after reverting) are, right?