Mark static_empty_bucket_ptr storage as const(expr) - #95
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Even though the returned pointer must be non-const, it would be a mistake if any code in this library ever wrote into it. Marking the storage const (and casting away the const-ness on return) is a clear way to indicate that. (It may even show the fault during unit testing in case such a bug was ever introduced.) Furthermore, when C++20 is enabled we can mark it constexpr, which means that the storage is defined as part of the compiled library's readonly data and doesn't need the atomic-initialize-upon-first-use guard variable. This also helps to avoid ODR hazards when linking this header-only library into multiple shared libraries (as can happen with nanobind when in NB_STATIC mode). While we're here, we might as well mark all static constants constexpr as a cleanup -- even those not needed by the bucket_entry code.
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Even though the returned pointer must be non-const, it would be a mistake if any code in this library ever wrote into it. Marking the storage const (and casting away the const-ness on return) is a clear way to indicate that. (It may even show the fault during unit testing in case such a bug was ever introduced.)
Furthermore, when C++20 is enabled we can mark it constexpr, which means that the storage is defined as part of the compiled library's readonly data and doesn't need the atomic-initialize-upon-first-use guard variable. This also helps to avoid ODR hazards when linking this header-only library into multiple shared libraries (as can happen with nanobind when in NB_STATIC mode).
While we're here, we might as well mark all static constants constexpr as a cleanup -- even those not needed by the bucket_entry code.