The use of "return true ? <object> : <exception>" syntax in code about constexpr HOT 4 OPEN

Well there is P1073R0 that we might have some day.

from constexpr.

This is Scott Schurr's link-time hack that attempts to prevent accidental non-constexpr usage of a function. The idea is that if you accidentally call a function in a non-constexpr context, it will reference a symbol that is not defined and cause a link error. Notice that everything that is thrown is externed, but not defined anywhere.

This is definitely not portable, but it works (worked?) on some compilers. I hope in future there will be better ways to guard against non-constexpr usage. The constexpr parts of C++ are evolving fast at the moment, so this repo is a dated experiment at this point.

from constexpr.

Unfortunately, this doesn't work on MSVC anymore.
cx_sha256.h(24,26): error C7631: 'cx::err::`anonymous-namespace'::sha256_runtime_error': variable with internal linkage declared but not defined

It's also a warning on clang (-Wundefined-internal) but that can be disabled.

This thread has some ideas - could we use one of those instead?
https://stackoverflow.com/questions/39650122/how-to-ensure-constexpr-function-never-called-at-runtime

from constexpr.

There is consteval in the C++20 standard now: https://en.cppreference.com/w/cpp/language/consteval

from constexpr.