I see what you are doing, and it seems to me the code could be simplified as follows:

• Instead of passing a diagnostics_t object, we pass a basic_json::reference to the exceptions (just as you did).
• But instead of using templates, that parameter could just default to nullptr; that is, a null JSON value.
• This value will never occur in the parent relation, so the diagnostic string will always be empty.

This will have the same code as you have on the call site, but may have a simplified diagnostics_t class. What do you think?

basic_json is a template, so a function taking basic_json::reference cannot be non-templated; there is no universal null JSON value. I think, the further simplification is quite possible, but in a different way. The diagnostics_t class can be eliminated; its primary functionality can be moved to diagnostics_message.