From 26595285d36be6b506e8888283c2b0da30778d70 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Wed, 28 Jun 2023 22:10:43 +0300
Subject: [PATCH] Add variant<>::uses_double_storage(). Refs #37.

---
 include/boost/variant2/variant.hpp   | 22 ++++++++++++
 test/Jamfile                         |  2 ++
 test/variant_uses_double_storage.cpp | 50 ++++++++++++++++++++++++++++
 3 files changed, 74 insertions(+)
 create mode 100644 test/variant_uses_double_storage.cpp

diff --git a/include/boost/variant2/variant.hpp b/include/boost/variant2/variant.hpp
index 7451eda..f0e8fef 100644
--- a/include/boost/variant2/variant.hpp
+++ b/include/boost/variant2/variant.hpp
@@ -920,6 +920,11 @@ template<class... T> struct variant_base_impl<true, true, T...>
 
         this->emplace_impl<J, U>( std::is_nothrow_constructible<U, A&&...>(), std::forward<A>(a)... );
     }
+
+    static constexpr bool uses_double_storage() noexcept
+    {
+        return false;
+    }
 };
 
 // trivially destructible, double buffered
@@ -978,6 +983,11 @@ template<class... T> struct variant_base_impl<true, false, T...>
 
         ix_ = J * 2 + i2;
     }
+
+    static constexpr bool uses_double_storage() noexcept
+    {
+        return true;
+    }
 };
 
 // not trivially destructible, single buffered
@@ -1068,6 +1078,11 @@ template<class... T> struct variant_base_impl<false, true, T...>
         st_.emplace( mp11::mp_size_t<J>(), std::move(tmp) );
         ix_ = J;
     }
+
+    static constexpr bool uses_double_storage() noexcept
+    {
+        return false;
+    }
 };
 
 // not trivially destructible, double buffered
@@ -1193,6 +1208,11 @@ template<class... T> struct variant_base_impl<false, false, T...>
 
         ix_ = J * 2 + i2;
     }
+
+    static constexpr bool uses_double_storage() noexcept
+    {
+        return true;
+    }
 };
 
 } // namespace detail
@@ -1703,6 +1723,8 @@ public:
 
     using variant_base::index;
 
+    using variant_base::uses_double_storage;
+
     // swap
 
 private:
diff --git a/test/Jamfile b/test/Jamfile
index 3c67277..f843257 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -131,3 +131,5 @@ local JSON = <library>/boost//json/<warnings>off "<toolset>msvc-14.0:<build>no"
 
 run variant_json_value_from.cpp : : : $(JSON) ;
 run variant_json_value_to.cpp : : : $(JSON) ;
+
+compile variant_uses_double_storage.cpp ;
diff --git a/test/variant_uses_double_storage.cpp b/test/variant_uses_double_storage.cpp
new file mode 100644
index 0000000..d78efc6
--- /dev/null
+++ b/test/variant_uses_double_storage.cpp
@@ -0,0 +1,50 @@
+// Copyright 2023 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+#include <type_traits>
+
+using namespace boost::variant2;
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+struct X1
+{
+};
+
+STATIC_ASSERT( std::is_nothrow_move_constructible<X1>::value );
+STATIC_ASSERT( std::is_trivially_destructible<X1>::value );
+
+struct X2
+{
+    ~X2() {}
+};
+
+STATIC_ASSERT( std::is_nothrow_move_constructible<X2>::value );
+STATIC_ASSERT( !std::is_trivially_destructible<X2>::value );
+
+struct X3
+{
+    X3( X3&& ) {}
+};
+
+STATIC_ASSERT( !std::is_nothrow_move_constructible<X3>::value );
+STATIC_ASSERT( std::is_trivially_destructible<X3>::value );
+
+struct X4
+{
+    ~X4() {}
+    X4( X4&& ) {}
+};
+
+STATIC_ASSERT( !std::is_nothrow_move_constructible<X4>::value );
+STATIC_ASSERT( !std::is_trivially_destructible<X4>::value );
+
+//
+
+STATIC_ASSERT( !variant<int, float>::uses_double_storage() );
+STATIC_ASSERT( !variant<int, float, X1>::uses_double_storage() );
+STATIC_ASSERT( !variant<int, float, X2>::uses_double_storage() );
+STATIC_ASSERT( variant<int, float, X3>::uses_double_storage() );
+STATIC_ASSERT( variant<int, float, X4>::uses_double_storage() );