Some libraries such as https://github.com/nlohmann/json have classes which
are in fact typedefs for very complicated types, for example:

template<template<typename U, typename V, typename... Args> class
> ObjectType =
> std::map,
> template<typename U, typename... Args> class ArrayType =
> std::vector,
> class StringType = std::string, class BooleanType = bool,
> class NumberIntegerType = std::int64_t,
> class NumberUnsignedType = std::uint64_t,
> class NumberFloatType = double,
> template<typename U> class AllocatorType = std::allocator,
> template<typename T, typename SFINAE = void> class JSONSerializer
> =
> adl_serializer,
> class BinaryType = std::vector<std::uint8_t>,
> class CustomBaseClass = void>
> class basic_json;
>
using json = basic_json<>;


I would like to forward declare their json class like this:

class json;


The problem is that if I try doing this, I get the following error:

nlohmann/json.hpp: error: conflicting declaration ‘using json = class
> basic_json<>’
>
main.cpp: note: previous declaration as ‘class json’


I don't see why this should be an error. class json; declares an incomplete
type called json and I can't access its members, instantiate it, etc anyway
unless I include its definition. So it shouldn't matter if it's a class, a
typedef for a class, or even a typedef for a primitive.

So I suggest that class json; simply means that I'm forward declaring an
incomplete type that can be anything, instead of meaning that it *must* be
a class (as opposed to a typedef for a class). Alternatively, a new syntax
such as typename json; could also work, as long as I don't need to copy the
entire declaration for their complicated template.