The standard libraries for some platforms declare (most) functions noexcept (possibly using compiler-specific notation). C++17 made the exception specification part of the type of the function. As a consequence, some compilers error in some cases where the exception specifications for redeclarations differ.

clang errors if the first declaration doesn't have an exception specification and a later one is noexcept. It doesn't complain in the reverse case. gcc seems to accept differences in either order. (That might be a gcc bug?) Visual Studio seems to reject any difference, regardless of include order.

This means that FORBID_C_FUNCTION needs to be able to provide a declaration that matches the exception specification of the corresponding library declaration. (It's easier to just match the library always, rather than further conditionalize on whether the compiler might complain.)