C++23 Compatibility
Implicit cast from rvalue references to lvalue references like the one in discard(T&&) -> T& is not allowed in C++23, so dune-grid fails to build on GCC 13 in C++23 mode. This MR is what I think is the fix.
Implicit cast from rvalue references to lvalue references like the one in discard(T&&) -> T& is not allowed in C++23, so dune-grid fails to build on GCC 13 in C++23 mode. This MR is what I think is the fix.