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. — Reply to this email directly, view it on GitHub <#24 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAK6QJWABV22VUVRPYKJM2LX6RFSHAVCNFSM6AAAAAAYKU2DOGVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMYTONJTGU3TCMBWG4> . You are receiving this because you authored the thread.Message ID: ***@***.***>

[firedtoad]

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