std::experimental has not been declared

The error message "std::experimental has not been declared" means that the compiler cannot find a definition for the experimental header file in the standard library. This header file contains experimental features that are not yet standardized and may not be available on all platforms.

To fix this error, you need to make sure that your compiler supports the experimental header file and that you have included it in your code. You can include the experimental header file using the following code:

#include <experimental/some_header_file>

Alternatively, you can use the non-experimental version of the header file if it is available in the standard library. For example, if you are using an experimental version of the filesystem header file, you can use the non-experimental version by including the following code:

#include <filesystem>

Keep in mind that experimental features may change or be removed in future versions of the standard, so it's important to check the documentation for the specific feature you are using and to test your code thoroughly.