[P4144R1](https://isocpp.org/files/papers/P4144R1.html) reverted [P2447R6](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2447r6.html) by removing `span`’s `initializer_list` constructor. We propose to add it back in C++29, but with constraints this time, in order to avoid the unfortunate conversions that led to its removal.