Hi Nathan3, glad you enjoyed the talk.
Compiler support for the C23 version of constexpr seems to be fairly new, but it appears that the C23 version of constexpr will be a strict subset of the constexpr features available in C++. C23 will permit some kinds of constexpr objects, but not constexpr functions. As such, some existing C++ code will be valid C23 code, making life easier for those who need their code to compile as either C or C++. We'll also be able to use constexpr objects where today we would use macros; that will be nice because (unlike evaluating a macro), evaluating a constexpr object can't produce side effects.
Unlike const objects, constexpr objects will be considered constant expressions in both C and C++. As such, you'll be able to use constexpr objects in contexts that require constant expressions, such as enum definitions:
int const cx1 = 10;
enum { e1 = cx1 }; // valid in C++, but not in C

 constexpr int cx2 = 20;
 enum { e2 = cx2 };    // valid in C++ and C23

 Hope that helps - let me know if you have additional questions.