What is the difference between std::span and gsl::span?

Question

In the 2010's, when std::span was not yet standardized, the C++ Core Guidelines project got started. It quickly developed a small library of constructs which authors found they need to recommend in order to be able to abide by the guidelines - the guidelines support library (GSL) (link is to Microsoft's implementation). Among other things, the GSL includes a span class: gsl::span<T>. Eventually, in 2020, std::span was officially standardized.

Recently, as I mentioned std::span to someone, they told me:

std::span is a trap, gsl::span should be used instead

what could they have meant? Isn't std::span supposed to be basically the same thing as gsl::span? I mean, after all, it's basically just a structure with a pointer and a size_t, and some convenience methods, isn't it?

Answer 1

From the GSL span class' documentation page (emphasis mine):

The only difference between gsl::span and std::span is that gsl::span strictly enforces runtime bounds checking. Any violations of the bounds check results in termination of the program.

and the same goes for the two span classes' iterators.