From e3745b20727e73b4b4575f9efadd53eadc4d0969 Mon Sep 17 00:00:00 2001
From: Glen Fernandes <glen.fernandes@gmail.com>
Date: Sat, 6 Aug 2022 18:53:40 -0400
Subject: [PATCH] Add constexpr support to empty_value

---
 include/boost/core/empty_value.hpp  | 32 +++++++-------
 test/Jamfile.v2                     |  1 +
 test/empty_value_constexpr_test.cpp | 65 +++++++++++++++++++++++++++++
 3 files changed, 82 insertions(+), 16 deletions(-)
 create mode 100644 test/empty_value_constexpr_test.cpp

diff --git a/include/boost/core/empty_value.hpp b/include/boost/core/empty_value.hpp
index 9dfd442..d8ffa30 100644
--- a/include/boost/core/empty_value.hpp
+++ b/include/boost/core/empty_value.hpp
@@ -56,37 +56,37 @@ public:
 #if !defined(BOOST_NO_CXX11_DEFAULTED_FUNCTIONS)
     empty_value() = default;
 #else
-    empty_value() { }
+    BOOST_CONSTEXPR empty_value() { }
 #endif
 
-    empty_value(boost::empty_init_t)
+    BOOST_CONSTEXPR empty_value(boost::empty_init_t)
         : value_() { }
 
 #if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
 #if !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)
     template<class U, class... Args>
-    empty_value(boost::empty_init_t, U&& value, Args&&... args)
+    BOOST_CONSTEXPR empty_value(boost::empty_init_t, U&& value, Args&&... args)
         : value_(std::forward<U>(value), std::forward<Args>(args)...) { }
 #else
     template<class U>
-    empty_value(boost::empty_init_t, U&& value)
+    BOOST_CONSTEXPR empty_value(boost::empty_init_t, U&& value)
         : value_(std::forward<U>(value)) { }
 #endif
 #else
     template<class U>
-    empty_value(boost::empty_init_t, const U& value)
+    BOOST_CONSTEXPR empty_value(boost::empty_init_t, const U& value)
         : value_(value) { }
 
     template<class U>
-    empty_value(boost::empty_init_t, U& value)
+    BOOST_CONSTEXPR empty_value(boost::empty_init_t, U& value)
         : value_(value) { }
 #endif
 
-    const T& get() const BOOST_NOEXCEPT {
+    BOOST_CONSTEXPR const T& get() const BOOST_NOEXCEPT {
         return value_;
     }
 
-    T& get() BOOST_NOEXCEPT {
+    BOOST_CXX14_CONSTEXPR T& get() BOOST_NOEXCEPT {
         return value_;
     }
 
@@ -104,37 +104,37 @@ public:
 #if !defined(BOOST_NO_CXX11_DEFAULTED_FUNCTIONS)
     empty_value() = default;
 #else
-    empty_value() { }
+    BOOST_CONSTEXPR empty_value() { }
 #endif
 
-    empty_value(boost::empty_init_t)
+    BOOST_CONSTEXPR empty_value(boost::empty_init_t)
         : T() { }
 
 #if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
 #if !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)
     template<class U, class... Args>
-    empty_value(boost::empty_init_t, U&& value, Args&&... args)
+    BOOST_CONSTEXPR empty_value(boost::empty_init_t, U&& value, Args&&... args)
         : T(std::forward<U>(value), std::forward<Args>(args)...) { }
 #else
     template<class U>
-    empty_value(boost::empty_init_t, U&& value)
+    BOOST_CONSTEXPR empty_value(boost::empty_init_t, U&& value)
         : T(std::forward<U>(value)) { }
 #endif
 #else
     template<class U>
-    empty_value(boost::empty_init_t, const U& value)
+    BOOST_CONSTEXPR empty_value(boost::empty_init_t, const U& value)
         : T(value) { }
 
     template<class U>
-    empty_value(boost::empty_init_t, U& value)
+    BOOST_CONSTEXPR empty_value(boost::empty_init_t, U& value)
         : T(value) { }
 #endif
 
-    const T& get() const BOOST_NOEXCEPT {
+    BOOST_CONSTEXPR const T& get() const BOOST_NOEXCEPT {
         return *this;
     }
 
-    T& get() BOOST_NOEXCEPT {
+    BOOST_CXX14_CONSTEXPR T& get() BOOST_NOEXCEPT {
         return *this;
     }
 };
diff --git a/test/Jamfile.v2 b/test/Jamfile.v2
index 373be22..3ce6b00 100644
--- a/test/Jamfile.v2
+++ b/test/Jamfile.v2
@@ -203,6 +203,7 @@ compile first_scalar_constexpr_test.cpp ;
 run empty_value_test.cpp ;
 run empty_value_size_test.cpp ;
 run empty_value_final_test.cpp ;
+run empty_value_constexpr_test.cpp ;
 
 run quick_exit_test.cpp ;
 run-fail quick_exit_fail.cpp ;
diff --git a/test/empty_value_constexpr_test.cpp b/test/empty_value_constexpr_test.cpp
new file mode 100644
index 0000000..f05e314
--- /dev/null
+++ b/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