Would anyone be interested in working on a proposal to add constexpr functionality to std::shared_ptr. An implementation based on libstdc++ is linked below:

https://github.com/SCT4SP/constexpr_shared_ptr

For convenience, `_GLIBCXX26_CONSTEXPR` and feature macro `__cpp_lib_constexpr_shared_ptr` have been defined and used throughout.

The implementation takes advantage of recent support in GCC and Clang for P2738 (constexpr cast from void*: towards constexpr type-erasure). Beyond that we have the usual avoidance of placement new with `allocator_traits::construct` etc.; `std::is_constant_evaluated` allows avoidance of atomic operations; and `std::make_shared`, `std::allocate_shared` and variants have required two separate allocations to avoid illegal pointer casting.

Comments welcome.

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Paul