I have the following code:

#include <span>

int* pointers[3];
std::span<const int* const> s = pointers;

GCC accepts this, but Clang (compiling libstdc++) rejects it, stating:

<source>:4:29: error: no viable conversion from 'int *[3]' to 'std::span<const int *const>'
    4 | std::span<const int* const> s = pointers;
      |                             ^   ~~~~~~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../include/c++/15.0.0/span:230:7: note: candidate constructor not viable: no known conversion from 'int *[3]' to 'const span<const int *const> &' for 1st argument
  230 |       span(const span&) noexcept = default;
      |       ^    ~~~~~~~~~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../include/c++/15.0.0/span:192:2: note: candidate template ignored: could not match 'const int *' against 'int *'
  192 |         span(type_identity_t<element_type> (&__arr)[_ArrayExtent]) noexcept
      |         ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../include/c++/15.0.0/span:199:2: note: candidate template ignored: could not match 'array<_Tp, _ArrayExtent>' against 'int *[3]'
  199 |         span(array<_Tp, _ArrayExtent>& __arr) noexcept
      |         ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../include/c++/15.0.0/span:206:2: note: candidate template ignored: could not match 'array<_Tp, _ArrayExtent>' against 'int *[3]'
  206 |         span(const array<_Tp, _ArrayExtent>& __arr) noexcept
      |         ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../include/c++/15.0.0/span:218:2: note: candidate template ignored: constraints not satisfied [with _Range = int *(&)[3]]
  218 |         span(_Range&& __range)
      |         ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../include/c++/15.0.0/span:213:8: note: because '!is_array_v<remove_cvref_t<int *(&)[3]> >' evaluated to false
  213 |           && (!is_array_v<remove_cvref_t<_Range>>)
      |               ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../include/c++/15.0.0/span:238:2: note: candidate template ignored: could not match 'span<_OType, _OExtent>' against 'int *[3]'
  238 |         span(const span<_OType, _OExtent>& __s) noexcept
      |         ^

See Compiler Explorer

I was under the impression that std::span can be constructed from ranges if only a qualification conversion (such as int* to const int * const) is required, so which compiler is correct here?

This is quite likely just a Clang/GCC interoperability quirk, considering that the code also compiles with -stdlib=libc++.