`<span>`: How cromulent is `span<int, static_cast<size_t>(-2)>`?

[StephanTLavavej]

Raised by A. Jiang (@frederick-vs-ja) in #5274 (comment).

span<int, static_cast<size_t>(-2)> is bogus, but how bogus? It clearly violates the size_bytes() precondition if called. It also violates all constructor preconditions. But can we static_assert when this type is instantiated? An LWG issue would be nice.

There's implementation divergence: https://godbolt.org/z/7ha8rK18K

#ifdef _MSC_VER
#include <crtdbg.h>
#endif

#include <cstddef>
#include <iostream>
#include <span>
using namespace std;

int main() {
#ifdef _MSC_VER
    _CrtSetReportMode(_CRT_ASSERT, _CRTDBG_MODE_FILE);
    _CrtSetReportFile(_CRT_ASSERT, _CRTDBG_FILE_STDOUT);
#endif

    constexpr size_t BogusSize = static_cast<size_t>(-2);

    int arr[3]{};
    span<int, BogusSize> sp(arr, BogusSize);  // lies
    cout << "Constructed span." << endl;
    cout << "sp.size_bytes(): " << sp.size_bytes() << endl;
}

• With VS 2022 17.10 (long before STL Hardening #5274) debug mode, size_bytes() emits a debug assertion:

  Program stdout
  Constructed span.
  Z:/compilers/msvc/14.40.33807-14.40.33811.0/include\span(435) : Assertion failed: size of span in bytes exceeds std::numeric_limits<size_t>::max()
  

• In STL Hardening #5274, I am static_asserting (unconditionally, regardless of debug mode) when any constructor is instantiated, but not the whole type.
• With GCC 14.2 libstdc++ debug mode, the constructor emits a debug assertion:

  Program stderr
  /opt/compiler-explorer/gcc-14.2.0/include/c++/14.2.0/span:165:
  In function:
      constexpr std::span<_Type, _Extent>::span(_It, size_type) [with _It = 
      int*; _Type = int; long unsigned int _Extent = 18446744073709551614; 
      size_type = long unsigned int]
  
  Error: function requires a valid iterator range [first, first + count).
  [...]
  

• With Clang 19.1.0 libc++, a compiler error is emitted due to the constructor taking an array:

  Compiler stderr
  In file included from <source>:7:
  /opt/compiler-explorer/clang-19.1.0/bin/../include/c++/v1/span:281:79: error: array is too large (18446744073709551614 elements)
    281 |   _LIBCPP_HIDE_FROM_ABI constexpr span(type_identity_t<element_type> (&__arr)[_Extent]) noexcept : __data_{__arr} {}
        |                                                                               ^~~~~~~
  <source>:19:26: note: in instantiation of template class 'std::span<int, 18446744073709551614>' requested here
     19 |     span<int, BogusSize> sp(arr, BogusSize);  // lies
        |                          ^