Can't build with nlohmann/json installed without -DJSON_MultipleHeaders=ON

[kou]

Does this issue affect the google-cloud-cpp project? If the problem is with
the Google Cloud service exposed by the google-cloud-cpp libraries instead of
the client libraries themselves, you may consider opening a support request
instead. The google-cloud-cpp developers cannot help you troubleshoot problems
with the service itself.

Yes.

What component of google-cloud-cpp is this related to? For example, is
this related to bigtable (i.e., something in google/cloud/bigtable), or GCS
(i.e., something in google/cloud/storage)?

google/cloud/internal/oauth2_universe_domain.h

Describe the bug A clear and concise description of what the bug is.

google-cloud-cpp/google/cloud/internal/oauth2_universe_domain.h

Line 22 in f54ba19

#include <nlohmann/json_fwd.hpp>

uses nlohmann/json_fwd.hpp not nlohmann/json.hpp but json_fwd.hpp is available only when nlohmann/json is installed with -DJSON_MultipleHeaders=ON.

https://github.com/nlohmann/json?tab=readme-ov-file#integration

You can further use file include/nlohmann/json_fwd.hpp for forward-declarations. The installation of json_fwd.hpp (as part of cmake's install step), can be achieved by setting -DJSON_MultipleHeaders=ON.

nlohmann/json provided by EPEL for RHEL 8 family doesn't provide json_fwd.hpp: https://packages.fedoraproject.org/pkgs/json/json-devel/epel-8.html#files

So we can't use the json-devel package on AlmaLinux 8. Here is an error log in apache/arrow:

https://github.com/ursacomputing/crossbow/actions/runs/9804176444/job/27071504390#step:8:3180

-- stderr output is:
In file included from /root/rpmbuild/BUILD/apache-arrow-17.0.0.dev465/cpp/build/google_cloud_cpp_ep-prefix/src/google_cloud_cpp_ep/google/cloud/internal/oauth2_authorized_user_credentials.cc:16:
/root/rpmbuild/BUILD/apache-arrow-17.0.0.dev465/cpp/build/google_cloud_cpp_ep-prefix/src/google_cloud_cpp_ep/google/cloud/internal/oauth2_universe_domain.h:22:10: fatal error: nlohmann/json_fwd.hpp: No such file or directory
 #include <nlohmann/json_fwd.hpp>
          ^~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.

To Reproduce Steps to reproduce the behavior:

1. Prepare RHEL 8 based distribution such as AlmaLinux 8
2. Prepare EPEL by sudo dnf install -y epel-release or something
3. Install nlohmann/json by sudo dnf install -y json-devel
4. Build
5. See error

Expected behavior A clear and concise description of what you expected to
happen.

google-cloud-cpp can be built with nlohmann/json installed without -DJSON_MultipleHeaders=ON.

Operating system: If you are using a Linux distribution please include the
name and version of the distribution too.

AlmaLinux 8.

What compiler and version are you using? Please include the output of
g++ -v or clang++ -v or the equivalent command-line flag.

https://github.com/ursacomputing/crossbow/actions/runs/9804176444/job/27071504390#step:8:1579

-- The C compiler identification is GNU 8.5.0
-- The CXX compiler identification is GNU 8.5.0

What version of google-cloud-cpp are you using? Please include the output
from git rev-parse HEAD if you are compiling from source, or the version
number from the applicable google/cloud/*/version.h file.

https://github.com/googleapis/google-cloud-cpp/releases/tag/v2.22.0

Additional context Add any other context about the problem here.

Can we use nlohmann/json.h instead for portability?

diff --git a/google/cloud/internal/oauth2_universe_domain.h b/google/cloud/internal/oauth2_universe_domain.h
index e74e438a0d..ca0fc6aaf8 100644
--- a/google/cloud/internal/oauth2_universe_domain.h
+++ b/google/cloud/internal/oauth2_universe_domain.h
@@ -19,7 +19,7 @@
 #include "google/cloud/status.h"
 #include "google/cloud/status_or.h"
 #include "google/cloud/version.h"
-#include <nlohmann/json_fwd.hpp>
+#include <nlohmann/json.hpp>
 #include <string>
 
 namespace google {

[coryan]

Thanks for this detailed bug report. As you probably noticed we merged a PR earlier today. FWIW, we stopped testing on RHEL 8 (and its brethren, such as AlmaLinux or RockyLinux). Happy to accept bug reports and suggested fixes like this one.

[kou]

Thanks!