Overload resolution ambiguity between std::data and boost::data on boost::span

[DiegoBaldassarMilleuno]

The following code fails to compile (in C++17 and up) because of an ambiguous resolution between std::data and boost::data (ADL-found):

#include <boost/core/span.hpp>
#include <iterator>

int* f(boost::span<int> spn) {
	using std::data;
	return data(spn);
}

Output of GCC15:

./test.cpp: In function 'int* f(boost::span<int>)':
./test.cpp:19:20: error: call of overloaded 'data(boost::span<int>&)' is ambiguous
   19 |         return data(spn);
      |                ~~~~^~~~~
./test.cpp:19:20: note: there are 8 candidates
In file included from /opt/homebrew/Cellar/gcc/15.2.0/include/c++/15/array:46,
                 from boost_1_89_0/boost/core/span.hpp:13,
                 from ./test.cpp:1:
/opt/homebrew/Cellar/gcc/15.2.0/include/c++/15/bits/range_access.h:335:5: note: candidate 1: 'constexpr decltype (__cont.data()) std::data(const _Container&) [with _Container = boost::span<int>; decltype (__cont.data()) = int*]'
  335 |     data(const _Container& __cont) noexcept(noexcept(__cont.data()))
      |     ^~~~
/opt/homebrew/Cellar/gcc/15.2.0/include/c++/15/bits/range_access.h:324:5: note: candidate 2: 'constexpr decltype (__cont.data()) std::data(_Container&) [with _Container = boost::span<int>; decltype (__cont.data()) = int*]'
  324 |     data(_Container& __cont) noexcept(noexcept(__cont.data()))
      |     ^~~~
In file included from boost_1_89_0/boost/core/span.hpp:12:
boost_1_89_0/boost/core/data.hpp:18:1: note: candidate 3: 'constexpr decltype (c.data()) boost::data(C&) [with C =span<int>; decltype (c.data()) = int*]'
   18 | data(C& c) noexcept(noexcept(c.data())) -> decltype(c.data())
      | ^~~~
boost_1_89_0/boost/core/data.hpp:25:1: note: candidate 4: 'constexpr decltype (c.data()) boost::data(const C&) [with C = span<int>; decltype (c.data()) = int*]'
   25 | data(const C& c) noexcept(noexcept(c.data())) -> decltype(c.data())
      | ^~~~

This would not occur with std::span.
Tested on Boost 1.89.0. This happens consistently with recent enough versions of GCC, clang or Apple clang.

A few possible solutions:

1. move span to boost::core:: #196;
2. Adding a couple of extra overloads to data for boost::span and const boost::span;
3. Replacing the definitions for boost::data in boost/core/data.hpp only in C++17 mode and up with a single using std::data; in the boost namespace.

[Lastique]

I think it is worth doing all three of the proposed actions. There may be other components in namespace boost that support data. E.g. boost::string_view comes to mind.

[Lastique]

Actually, span-specific overloads should not be needed once boost::data and boost::size are converted to using-declarations. Moving boost::span into a deeper namespace is still worthwhile.

[Lastique]

@DiegoBaldassarMilleuno See if #207 fixes the problem.

[DiegoBaldassarMilleuno]

@Lastique Sorry for the late response, just tested commit 45bd9bf from develop and everything works fine on my end.