How can I exclude a part of code path in a method?

[aka-nse]

Hello.

In some cases, it is possible to have functions with code paths which are never executed.
For example, such as following:

public static long DoSomething(nint x)
{
    switch(Unsafe.SizeOf<nint>())
    {
    case 4:
        // do something for 32bit environment
        return ResultFor32;
    case 8:
        // do something for 64bit environment
        return ResultFor64;
    default:
        throw new NotSupportedException();
    }
}

Because size of nint in every dotnet runtime must be 4 or 8, therefore default path of switch can never be run.
This method aims for high performance using JIT optimization, so it is not possible to refactor this conditional branch into any design pattern.
However I want test the case of Unsafe.SizeOf<nint>() == 4 and Unsafe.SizeOf<nint>() == 8.
In such a case, is there a way to avoid to measure coverage only the specified code path?