fix const value return by emplace iterator

[firedtoad]

with const this code will not compile tsl::sparse_map<uint32_t,uint32_t> spp; const auto it=spp.emplace(1,1); it.first->second=1;

[Tessil]

Thank you for the contribution, unfortunately casting a std::pair<K, V>& to a std::pair<const K, V>& is an undefined behaviour. See https://stackoverflow.com/questions/8305793/is-it-possible-to-cast-a-pairkey-value-to-a-pairconst-key-value for more information.

[firedtoad]

It is not undefined behaviour, because they have the same memory layout. I use this modified version in my project. And it passed the clang && gcc UBsan！

Thibaut Goetghebuer-Planchon @.***> 于2023年10月10日周二 03:25写道：

Casting a std::pair<K, V>& to a std::pair<const K, V>& is an undefined behaviour. See https://stackoverflow.com/questions/8305793/is-it-possible-to-cast-a-pairkey-value-to-a-pairconst-key-value for more information.

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[firedtoad]

The new standard shows that it is layout compatible. https://godbolt.org/z/dKn99YTeM