Add asserts to span

test/Jamfile.v2

@@ -409,6 +409,7 @@ run span_test.cpp ;
 run span_types_test.cpp ;
 run span_constructible_test.cpp ;
 run span_deduction_guide_test.cpp ;
+run span_constexpr_test.cpp ;
 run as_bytes_test.cpp ;
 run as_writable_bytes_test.cpp ;
 compile span_boost_begin_test.cpp ;

test/span_constexpr_test.cpp

@@ -0,0 +1,72 @@
+/*
+Copyright 2025 Glen Joseph Fernandes
+(glenjofe@gmail.com)
+
+Distributed under the Boost Software License, Version 1.0.
+(http://www.boost.org/LICENSE_1_0.txt)
+*/
+#include <boost/config.hpp>
+#if !defined(BOOST_NO_CXX11_CONSTEXPR)
+#include <boost/core/span.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+constexpr const int array[4]{ 5, 10, 15, 20 };
+
+void test_first()
+{
+    constexpr boost::span<const int> s =
+        boost::span<const int>(array, 4).first(2);
+    BOOST_TEST_EQ(s.data(), &array[0]);
+    BOOST_TEST_EQ(s.size(), 2);
+}
+
+void test_last()
+{
+    constexpr boost::span<const int> s =
+        boost::span<const int>(array, 4).last(2);
+    BOOST_TEST_EQ(s.data(), &array[2]);
+    BOOST_TEST_EQ(s.size(), 2);
+}
+
+void test_subspan()
+{
+    constexpr boost::span<const int> s =
+        boost::span<const int>(array, 4).subspan(1, 2);
+    BOOST_TEST_EQ(s.data(), &array[1]);
+    BOOST_TEST_EQ(s.size(), 2);
+}
+
+void test_index()
+{
+    constexpr const int i = boost::span<const int>(array, 4)[1];
+    BOOST_TEST_EQ(i, 10);
+}
+
+void test_front()
+{
+    constexpr const int i = boost::span<const int>(array, 4).front();
+    BOOST_TEST_EQ(i, 5);
+}
+
+void test_back()
+{
+    constexpr const int i = boost::span<const int>(array, 4).back();
+    BOOST_TEST_EQ(i, 20);
+}
+
+int main()
+{
+    test_first();
+    test_last();
+    test_subspan();
+    test_index();
+    test_front();
+    test_back();
+    return boost::report_errors();
+}
+#else
+int main()
+{
+    return 0;
+}
+#endif