Relative includes of json_fwd.hpp in detail/meta.hpp. [Develop branch]

[przemkovv]

Bug Report

• What is the issue you have?
  I have nlohmann/json library added using ExternalProject_Add in CMakeLists.txt.
  In my project, the <INSTALL_DIR>/include/ folder is added as the system include directories.
  During compilation, I've an error of missing json_fwd.hpp included from detail/meta.cpp.
  The file json_fwd.hpp is "missing" because it is located in include/nlohmann and only include/ is added to 'global' include path.

I'm using JSON_MultipleHeaders=ON flag in CMake.

• Please describe the steps to reproduce the issue. Can you provide a small but working code example?
  File CMakeLists.txt

cmake_minimum_required(VERSION 3.10)
project(jj)

include(ExternalProject)
ExternalProject_Add(json
    GIT_REPOSITORY https://github.com/nlohmann/json.git
    GIT_TAG develop
    INSTALL_DIR ${PROJECT_BINARY_DIR}
    CMAKE_ARGS -DCMAKE_INSTALL_PREFIX:PATH=<INSTALL_DIR> -DBUILD_TESTING=0 -DJSON_MultipleHeaders=ON
    UPDATE_COMMAND ""
    )

include_directories(SYSTEM ${PROJECT_BINARY_DIR}/include)
add_executable(jj src/test.cpp)
add_dependencies(jj json)

File src/test.cpp:

#include <nlohmann/json.hpp>

int main() {
  return 0;
}

• What is the expected behavior?
  json_fwd.hpp should be visible for files in detail/ of the json library.

• And what is the actual behavior instead?
  The relative path in meta.cpp to the json_fwd.hpp doesn't allow to compile.
  During compilation, I've got the following error:

In file included from /home/przemkovv/projects/sandbox/json_bug/src/test.cpp:2:
In file included from /home/przemkovv/projects/sandbox/json_bug/build/include/nlohmann/json.hpp:46:
/home/przemkovv/projects/sandbox/json_bug/build/include/nlohmann/detail/meta.hpp:9:10: fatal error: 'json_fwd.hpp' file not found
#include "json_fwd.hpp"
         ^~~~~~~~~~~~~~

• Which compiler and operating system are you using? Is it a supported compiler?
  Arch Linux, Clang 7.0.0

• Did you use a released version of the library or the version from the develop branch?
  develop

• If you experience a compilation error: can you compile and run the unit tests?
  Cannot compile.

[nlohmann]

Related: #923, #925

[theodelrieu]

I believe we should rename the develop folder to have the same name as the installation target: nlohmann.

Then, stop using relative includes in the sources (they were only useful with the C++ amalgamate tool).

We have to keep including json.hpp (no prefix) in the tests for now, to switch easily between amalgamated/non-amalgamated versions.

I don't know how the python script decides which headers to include, but if it has the same logic about ""-quoted headers, we have to modify it to only include <nlohmann/*> headers.

[nlohmann]

The issue could be solved by changing the include line from

include_directories(SYSTEM ${PROJECT_BINARY_DIR}/include)

to

include_directories(SYSTEM ${PROJECT_BINARY_DIR}/include ${PROJECT_BINARY_DIR}/include/nlohmann)

[nlohmann]

Forget my proposal. This will be fixed properly with #944.

[przemkovv]

Thanks! Your library is always awesome. )