From 24a8174ef1c9b56226e5478ad65a913e28b79bda Mon Sep 17 00:00:00 2001
From: Glen Fernandes <glen.fernandes@gmail.com>
Date: Thu, 16 Jan 2025 19:12:31 -0500
Subject: [PATCH] Add asserts to span

---
 include/boost/core/detail/assert.hpp | 24 ++++++++++
 include/boost/core/span.hpp          | 19 +++++---
 test/Jamfile.v2                      |  1 +
 test/span_constexpr_test.cpp         | 72 ++++++++++++++++++++++++++++
 4 files changed, 109 insertions(+), 7 deletions(-)
 create mode 100644 include/boost/core/detail/assert.hpp
 create mode 100644 test/span_constexpr_test.cpp

diff --git a/include/boost/core/detail/assert.hpp b/include/boost/core/detail/assert.hpp
new file mode 100644
index 0000000..a5c2108
--- /dev/null
+++ b/include/boost/core/detail/assert.hpp
@@ -0,0 +1,24 @@
+/*
+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)
+*/
+#ifndef BOOST_CORE_DETAIL_ASSERT_HPP
+#define BOOST_CORE_DETAIL_ASSERT_HPP
+
+#include <cassert>
+
+#if !defined(NDEBUG) && \
+    !defined(__clang__) && \
+    !defined(__INTEL_COMPILER) && \
+    defined(__GNUC__) && \
+    (__GNUC__ < 5)
+#define BOOST_CORE_ASSERT(expr) \
+   ((expr) ? void(0) : __assert_fail(#expr, __FILE__, __LINE__, 0))
+#else
+#define BOOST_CORE_ASSERT(expr) assert(expr)
+#endif
+
+#endif
diff --git a/include/boost/core/span.hpp b/include/boost/core/span.hpp
index 2e64262..6732368 100644
--- a/include/boost/core/span.hpp
+++ b/include/boost/core/span.hpp
@@ -8,6 +8,7 @@ Distributed under the Boost Software License, Version 1.0.
 #ifndef BOOST_CORE_SPAN_HPP
 #define BOOST_CORE_SPAN_HPP
 
+#include <boost/core/detail/assert.hpp>
 #include <boost/core/data.hpp>
 #include <array>
 #include <iterator>
@@ -274,17 +275,21 @@ public:
     }
 
     constexpr span<T, dynamic_extent> first(size_type c) const {
-        return span<T, dynamic_extent>(s_.p, c);
+        return BOOST_CORE_ASSERT(c <= size()),
+            span<T, dynamic_extent>(s_.p, c);
     }
 
     constexpr span<T, dynamic_extent> last(size_type c) const {
-        return span<T, dynamic_extent>(s_.p + (s_.n - c), c);
+        return BOOST_CORE_ASSERT(c <= size()),
+            span<T, dynamic_extent>(s_.p + (s_.n - c), c);
     }
 
     constexpr span<T, dynamic_extent> subspan(size_type o,
         size_type c = dynamic_extent) const {
-        return span<T, dynamic_extent>(s_.p + o,
-            c == dynamic_extent ? s_.n - o : c);
+        return BOOST_CORE_ASSERT(o <= size() &&
+                (c == dynamic_extent || c + o <= size())),
+            span<T, dynamic_extent>(s_.p + o,
+                c == dynamic_extent ? s_.n - o : c);
     }
 
     constexpr size_type size() const noexcept {
@@ -300,15 +305,15 @@ public:
     }
 
     constexpr reference operator[](size_type i) const {
-        return s_.p[i];
+        return BOOST_CORE_ASSERT(i < size()), s_.p[i];
     }
 
     constexpr reference front() const {
-        return *s_.p;
+        return BOOST_CORE_ASSERT(!empty()), *s_.p;
     }
 
     constexpr reference back() const {
-        return s_.p[s_.n - 1];
+        return BOOST_CORE_ASSERT(!empty()), s_.p[s_.n - 1];
     }
 
     constexpr pointer data() const noexcept {
diff --git a/test/Jamfile.v2 b/test/Jamfile.v2
index c8d302f..593b527 100644
--- a/test/Jamfile.v2
+++ b/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 ;
diff --git a/test/span_constexpr_test.cpp b/test/span_constexpr_test.cpp
new file mode 100644
index 0000000..c24751b
--- /dev/null
+++ b/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