Add a test for boost::span verifying unqualified calls to data()/size().

The test checks that unqualified calls to data()/size() with boost::span
as argument don't cause ambiguity between std:: and boost:: implementations.

Fixes https://github.com/boostorg/core/issues/206.

test/Jamfile.v2

@@ -416,6 +416,7 @@ run span_constexpr_test.cpp ;
 run as_bytes_test.cpp ;
 run as_writable_bytes_test.cpp ;
 compile span_boost_begin_test.cpp ;
+compile span_nonmem_data_size_test.cpp ;
 run make_span_test.cpp ;
 
 run splitmix64_test.cpp

test/span_nonmem_data_size_test.cpp

@@ -0,0 +1,44 @@
+/*
+ *             Copyright Andrey Semashev 2025.
+ * Distributed under the Boost Software License, Version 1.0.
+ *    (See accompanying file LICENSE_1_0.txt or copy at
+ *          http://www.boost.org/LICENSE_1_0.txt)
+ */
+/*
+ * The test verifies that unqualified calls to data() and size()
+ * don't cause ambiguity between std:: and boost:: implementations.
+ * The ambiguity used to be caused by ADL bringing boost::data()/size()
+ * due to namespace of boost::span and a using-declaration of
+ * std::data()/size().
+ *
+ * https://github.com/boostorg/core/issues/206
+ */
+
+#include <boost/config.hpp>
+
+#if !defined(BOOST_NO_CXX11_CONSTEXPR) && !defined(BOOST_NO_CXX11_DECLTYPE)
+
+#include <boost/core/span.hpp>
+#include <iterator>
+
+// Note: This preprocessor check should be equivalent to those in boost/core/data.hpp and boost/core/size.hpp
+#if (defined(__cpp_lib_nonmember_container_access) && (__cpp_lib_nonmember_container_access >= 201411l)) || \
+    (defined(_MSC_VER) && (_MSC_VER >= 1900))
+
+#include <boost/core/data.hpp>
+#include <boost/core/size.hpp>
+
+int* test_data_ambiguity(boost::span<int> sp)
+{
+    using std::data;
+    return data(sp);
+}
+
+boost::span<int>::size_type test_size_ambiguity(boost::span<int> sp)
+{
+    using std::size;
+    return size(sp);
+}
+
+#endif // (defined(__cpp_lib_nonmember_container_access) ...
+#endif // !defined(BOOST_NO_CXX11_CONSTEXPR) && !defined(BOOST_NO_CXX11_DECLTYPE)