If one wishes to be pedantic, constexpr is a C++-like language for generating C++ values at compile time and templates are a quite different language for doing the same thing (and some other things). The pitch of the comptime feature is that you can do all of the things you would do with templates and constexpr (and more things due to some limitations being relaxed and type introspection existing) in the language that is similar to the language you use for runtime code. You don't have to do anything in a weird metalanguage.
C++ has been trying to get compile time introspection and synthesis into the language for a while. There is a proposal going slowly through standardization, but it is not yet know when it will be ready (I believe it missed 2 or more release trains already).
Circle, mentioned elsewhere, is a C++ dialect with fully working compile time metaprogramming.
The big thing is the macro language is restricted to generating additional code when it's evaluated whereas compile time expressions are more about reducing expressions into literals.
Not any code that you can program yourself. It's a bunch of template deduction/instantiation rules specified in the C++ standard, baked into the compiler, which you can steer only to the extent that you can grab the template declaration syntax by whatever horns it it is trying to gore you with.
