unused-parameter on OSX when Diagnostics is off

[bflorac]

What is the issue you have?

When compiling on OSX (CLANG) the default compiler state has unused-parameter turn on. This issues warnings for json.hpp, class exception, function diagnostics(), because the leaf_element parameter is not used. A simple (void)(leaf_element) in the #ELSE clause will resolve the issue.

Please describe the steps to reproduce the issue.

Can you provide a small but working code example?

What is the expected behavior?

And what is the actual behavior instead?

Which compiler and operating system are you using?

• Compiler: CLANG
• Operating system: OSX

Which version of the library did you use?

• [x ] latest release version 3.9.1
• other release - please state the version: ___
• the develop branch

If you experience a compilation error: can you compile and run the unit tests?

• yes
• no - please copy/paste the error message below

[gregmarr]

Same as #2646
@nlohmann This should be a simple fix.

[eike-fokken]

If you are ok with using standard specific macros, you could also use
the [[maybe_unused]] specifier for c++17 and later.
It has the (albeit small) advantage that nothing unneccessary is done at runtime even when all optimization is turned off.

[nlohmann]

Duplicate of #2646.