C++26: more constexpr in the core language

Summary

Since constexpr was added to the language in C++11, its scope has been gradually expanded. In the beginning, we couldn’t even use if, else or loops, which were changed in C++14. C++17 added support for constexpr lambdas. C++20 added the ability to use allocation and use std::vector and std::string in constant expressions. In this article, let’s see how constexpr evolves with C++26. To be more punctual, let’s see what language features become more constexpr-friendly. We’ll discuss library changes in a separate article, as well as constexpr exceptions, which need both language and library changes.P2738R1: constexpr cast from void*Thanks to the acceptance of P2738R1, starting from C++26, one can cast from void* to a pointer of type T in constant expressions, if the type of the object at that adress is exactly the type of T.Note that conversions to interconvertible - including pointers to base classes - or not related types are not permitted.The motivation behind this change is to make several standard library functions or types work at compile time. To name a few examples: std::format, std::function, std::function_ref, std::any. The reason why this change will allow many more for more constexpr in the standard library is that storing void* is a commonly used compilation firewall technique to reduce template instantiations and the number of symbols in compiled binaries.P2747R2: constexpr placement newAs std::construct_at is a limited tool that only allows to perform value initialization but not others such as default or list initialization, there has been a need to make placement new usable in constant expressions.At the same time, placement new is a very, maybe even too flexible tool and to use it in a safe way requires casting to void* and then back to T*. This faced some issues, but the acceptance of P2738R1 and the ability of casting from void* in constant expressions made the impossible possible.If you are looking for more details, check P2747R2.P2686R5: constexpr ...