Add constexpr support to empty_value

test/empty_value_constexpr_test.cpp

@@ -0,0 +1,65 @@
+/*
+Copyright 2022 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/empty_value.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+struct empty {
+    constexpr int value() const {
+        return 1;
+    }
+};
+
+class type {
+public:
+    explicit constexpr type(int count)
+        : value_(count) { }
+
+    constexpr int value() const {
+        return value_;
+    }
+
+private:
+    int value_;
+};
+
+void test_int()
+{
+    constexpr boost::empty_value<int> v(boost::empty_init_t(), 4);
+    constexpr int c = v.get();
+    BOOST_TEST_EQ(c, 4);
+}
+
+void test_empty()
+{
+    constexpr boost::empty_value<empty> v = boost::empty_init_t();
+    constexpr int c = v.get().value();
+    BOOST_TEST_EQ(c, 1);
+}
+
+void test_type()
+{
+    constexpr boost::empty_value<type> v(boost::empty_init_t(), 2);
+    constexpr int c = v.get().value();
+    BOOST_TEST_EQ(c, 2);
+}
+
+int main()
+{
+    test_int();
+    test_empty();
+    test_type();
+    return boost::report_errors();
+}
+#else
+int main()
+{
+    return 0;
+}
+#endif