[pragma_x]

Since you've done this for real in a library, I have to ask: how would you decide to use a compile-time template solution like this versus a code generator or some other "outboard" tool to generate code?

I'm curious since I've gone back and forth on this in my own career. Both approaches come with their own pros and cons, but each get us to the same place.

[beached_whale]

I def have more experience with the compile time and it's big issue is how long it can take, but I think it can be overblown on the cost too. However, one can often mitigate this with good code separation and firewalling. The compile time methods will never(probably) be faster than generated code in the C++ compiler. But, a TU with a simple function like `Config parse_config( std::string_view json_doc );` can protect the other code and reduce the general compile time when working on the real code. I think both have their debugging issue with codegen being difficult to tie to the source and the compile time parts relying on the limited error facilities available currently. The proposed p2471 - https://wg21.link/p2471 should help with this a lot and allow for non-literal static_assert messages.

One place I have used the compile time, since JSON Link uses declarative mappings and is type based, is allowing for automatic (de)serialization of function calls over RPC/IPC. This makes things like communicating with web engines really neat, or doing serialized RPC calls over some other mechanism. Code like `server.register( "path", [] ( int x, Foo foo ) { ... } );` is working now in C++, and that's really neat and closely analogues some of the higher level API's in other languages.