From 4126ca650cb318f034420fac69b9101e16bc13c1 Mon Sep 17 00:00:00 2001
From: Adder <adder_2003@yahoo.com>
Date: Tue, 7 Jan 2025 00:43:06 +0200
Subject: [PATCH] `integral_constant`: As per `std`: Add `operator()`.

In the C++ Standard Library, `std::integral_constant` has `operator()`:

https://timsong-cpp.github.io/cppwp/meta.help

We hereby add `operator()` for the Boost version.
---
 .../boost/type_traits/integral_constant.hpp   |  2 ++
 test/integral_constant_test.cpp               | 28 +++++++++++++++++++
 2 files changed, 30 insertions(+)
 create mode 100755 test/integral_constant_test.cpp

diff --git a/include/boost/type_traits/integral_constant.hpp b/include/boost/type_traits/integral_constant.hpp
index 284ed02..7424c86 100644
--- a/include/boost/type_traits/integral_constant.hpp
+++ b/include/boost/type_traits/integral_constant.hpp
@@ -64,6 +64,7 @@ namespace boost{
          return *static_cast<const mpl::integral_c<T, val>*>(pdata);
       }
       BOOST_CONSTEXPR operator T()const BOOST_NOEXCEPT { return val; }
+      BOOST_CONSTEXPR T operator()()const BOOST_NOEXCEPT { return val; }
    };
 
    template <class T, T val>
@@ -84,6 +85,7 @@ namespace boost{
          return *static_cast<const mpl::bool_<val>*>(pdata);
       }
       BOOST_CONSTEXPR operator bool()const BOOST_NOEXCEPT { return val; }
+      BOOST_CONSTEXPR bool operator()()const BOOST_NOEXCEPT { return val; }
    };
 
    template <bool val>
diff --git a/test/integral_constant_test.cpp b/test/integral_constant_test.cpp
new file mode 100755
index 0000000..6938d5d
--- /dev/null
+++ b/test/integral_constant_test.cpp
@@ -0,0 +1,28 @@
+// Copyright 2025 DoctorNoobingstoneIPresume
+// 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)
+
+#include <boost/type_traits/integral_constant.hpp>
+#include <boost/type_traits/is_same.hpp>
+#include <boost/static_assert.hpp>
+
+int main ()
+{
+    // [2025-01-07] Adapted from example in https://en.cppreference.com/w/cpp/types/integral_constant ...
+
+    using two_t  = boost::integral_constant <int, 2>;
+    using four_t = boost::integral_constant <int, 4>;
+
+    BOOST_STATIC_ASSERT ((! boost::is_same <two_t, four_t>::value));
+    BOOST_STATIC_ASSERT ((two_t::value * 2 == four_t::value));
+    BOOST_STATIC_ASSERT ((two_t () << 1 == four_t ()));
+    BOOST_STATIC_ASSERT ((two_t () () << 1 == four_t () ()));
+
+    enum class E {e0, e1};
+    using c0 = boost::integral_constant <E, E::e0>;
+    using c1 = boost::integral_constant <E, E::e1>;
+    BOOST_STATIC_ASSERT ((c0::value != E::e1));
+    BOOST_STATIC_ASSERT ((c0 () == E::e0));
+    BOOST_STATIC_ASSERT ((boost::is_same <c1, c1>::value));
+}