Compiler warning: 'tuple_size' defined as a class template here but previously declared as a struct template

[abitmore]

Build Error Description

When cross-building macOS binaries (with clang), got this warning:

/home/ubuntu/build/bitshares/libraries/fc/include/fc/container/zeroed_array.hpp:78:4: warning: 'tuple_size' defined as a class template here but previously declared as a struct template [-Wmismatched-tags]
   class tuple_size< fc::zero_initialized_array< T, N > > : public tuple_size< array< T, N > > {};
   ^
/home/ubuntu/build/osxcross-e0a171828a72a0d7ad4409489033536590008ebf/target/bin/../SDK/MacOSX10.15.sdk/usr/include/c++/v1/__tuple:25:22: note: did you mean class here?
template <class _Tp> struct _LIBCPP_TEMPLATE_VIS tuple_size;
                     ^
1 warning generated.

Here is a related discussion with a suggested fix: nlohmann/json#1401.

Build Environment
Details about the build environment, including the relevant required libraries. Much of this information can be found in the CMakeFiles/CMakeOutput.log.

• Host OS: [e.g. Ubuntu 18.04 LTS]
• Host Physical RAM [e.g. 4GB]
• Source Branch/Tag: [e.g. master or 2.0.180425]
• OpenSSL Version: [e.g. 1.1.0g]
• Boost Version: [e.g. 1.65.1]
• C++ Compiler: [e.g. gcc version 4.8.5]

Steps To Reproduce
Steps to reproduce the behavior (example outlined below):

1. Using installation guide from this URL...
2. This is my complete build script...
3. It fails at this step with the following output...
4. See the error in the console log below...

Console Logs (optional)
Please provide the full console log, including all commands entered and their output. This will allow detailed troubleshooting.

CORE TEAM TASK LIST

• Evaluate Build Error
• Provide build guidance
• Create Bug Report

[abitmore]

Will be fixed by bitshares/bitshares-fc#202.

[abitmore]

PR bitshares/bitshares-fc#202 was merged, and FC was bumped in #2204, closing this issue.