warning C4068: unknown pragma 'GCC' on MSVC/cl

[MHebes]

What is the issue you have?

When compiling with MSVC, The following warnings are created:

warning C4068: unknown pragma 'GCC'

Please describe the steps to reproduce the issue.

1. Compile any program including json.hpp with cl
2. Observe warnings

Can you provide a small but working code example?

// main.cpp
#include "nlohmann/json.hpp"

int main() { return 0; }

$ cl /EHsc main.cpp

What is the expected behavior?

No warning should appear.

And what is the actual behavior instead?

MSVC warning C4068 appears.

Which compiler and operating system are you using?

• Compiler: cl /?, Microsoft (R) C/C++ Optimizing Compiler Version 19.27.29111 for x86 using the Developer Command Prompt for VS 2019.
• Operating system: Windows 10

Which version of the library did you use?

• 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

Workaround:

Adding

#pragma warning ( disable: 4068 )

to the top of either file disables the warning, or wrap all #pragma GCC ...s in #if defined(__clang__).

[MHebes]

Introduced in #2911 I believe.

[nlohmann]

I can reproduce the issue: the warnings appear in the CI, see https://github.com/nlohmann/json/runs/3300581735?check_suite_focus=true.