json_fwd.hpp doesn't work #3796

Description

json_fwd.hpp no longer works. When using it in a header there is an "incomplete class" error in the corresponding code file.

I have already figured out why this happens: The basic_json template differs between json_fwd.hpp and json.hpp. In json_fwd.hpp there is a class CustomBaseClass = void at the end which is not there in json.hpp (line 151 in json_fwd.hpp). When I manually remove that line in my project it works fine.

Reproduction steps

Just #include <nlohmann/json_fwd.hpp> in a header and try to use it in the code file.

Expected vs. actual results

Expected: Should work to use json_fwd.hpp to forward-declare nlohmann::json.
Actual: Does not work to forward-declare using json_fwd.hpp; results in "incomplete class" error in code file when trying to use it.

Minimal code example

No response

Error messages

error C2976: 'nlohmann::json_abi_v3_11_2::basic_json': too few template arguments
error C3203: 'basic_json': unspecialized class template can't be used as a template argument for template parameter 'T', expected a real type

Compiler and operating system

Windows, Microsoft Visual C++ 2019

Library version

3.11.2

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.