Can't create a std::span from a gsl::span anymore after implementing P1394r4

[ldionne]

Since P1394r4, std::span changed the following constructors:

template<class Container> constexpr span(Container&);
template<class Container> constexpr span(const Container&);

into the following constructor:

template <class R> constexpr span(R&&);

where R must be a std::contiguous_range. However, as it happens, gsl::span does not qualify as a contiguous_range because its iterators are not contiguous_iterators. As a result, the following code starts failing when a Standard Library implements the resolution of P1394r4:

cat <<EOF | clang++ -xc++ - -std=c++20 -fsyntax-only
#include <gsl/span>
#include <span>
gsl::span<int> gsl_span;
std::span<int> std_span = gsl_span;
EOF

Here is a reproducer on Godbolt: https://godbolt.org/z/c1b686vjh. This worked previously with libc++ before we implemented P1394r4.

I would suggest that gsl::span's iterators be made contiguous_iterators in C++20, which would improve its interoperability with standard library types.

Note that this issue was found while building a large code base with the latest version of libc++ -- I suspect this issue is going to hit a decent number of people when LLVM 14 is released.

[dmitrykobets-msft]

Hi @ldionne, thanks for finding this. Will investigate.

[dmitrykobets-msft]

Related to #982