Zig vs D generics

[Kagamin]

>In contrast, C++ code attempting to call non-constexpr functions 
>(i.e. functions only accessible at run time) in a constexpr 
>context (i.e. functions which are available both at compile time 
>and run time) will give an error at the call site.

Eh? So can C++ support an allocator that works both at compile 
time and at run time? I use such allocator in my D code so it's 
ctfeable.