Add test/variant_visit_cx.cpp. Refs #47.

include/boost/variant2/variant.hpp

@@ -2125,7 +2125,7 @@ template<class R, class F, class V1> struct visit_L1
     F&& f;
     V1&& v1;
 
-    template<class I> auto operator()( I ) const -> Vret<R, F, V1>
+    template<class I> constexpr auto operator()( I ) const -> Vret<R, F, V1>
     {
         return std::forward<F>(f)( unsafe_get<I::value>( std::forward<V1>(v1) ) );
     }

test/Jamfile

@@ -139,3 +139,5 @@ run variant_json_value_to.cpp : : : $(JSON) ;
 compile variant_uses_double_storage.cpp ;
 
 run variant_derived_construct2.cpp ;
+
+compile variant_visit_cx.cpp : [ requires cxx14_constexpr ] ;

test/variant_visit_cx.cpp

@@ -0,0 +1,51 @@
+// Copyright 2017, 2024 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/variant2/variant.hpp>
+using namespace boost::variant2;
+
+#if !defined(BOOST_MP11_HAS_CXX14_CONSTEXPR)
+
+#include <boost/config/pragma_message.hpp>
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr visit test because BOOST_MP11_HAS_CXX14_CONSTEXPR is not defined")
+
+int main() {}
+
+#else
+
+struct X
+{
+    constexpr operator int() const { return 2; }
+};
+
+struct F
+{
+    constexpr int operator()( int x ) const
+    {
+        return x;
+    }
+};
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+int main()
+{
+    {
+        constexpr variant<int> v( 1 );
+        STATIC_ASSERT( visit( F(), v ) == 1 );
+    }
+
+    {
+        constexpr variant<char, int> v( 'a' );
+        STATIC_ASSERT( visit( F(), v ) == 'a' );
+    }
+
+    {
+        constexpr variant<char, int, X> v( X{} );
+        STATIC_ASSERT( visit( F(), v ) == 2 );
+    }
+}
+
+#endif