From 3fc75407332c8117d6fb281ab7e32667830db97a Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Thu, 9 Dec 2021 18:34:02 +0200
Subject: [PATCH] Only define operator<< when all contained types are
 streamable (refs #31)

---
 include/boost/variant2/variant.hpp | 12 +++++++++-
 test/Jamfile                       |  1 +
 test/is_output_streamable.cpp      | 38 ++++++++++++++++++++++++++++++
 3 files changed, 50 insertions(+), 1 deletion(-)
 create mode 100644 test/is_output_streamable.cpp

diff --git a/include/boost/variant2/variant.hpp b/include/boost/variant2/variant.hpp
index 77d5d16..d4d3d82 100644
--- a/include/boost/variant2/variant.hpp
+++ b/include/boost/variant2/variant.hpp
@@ -2253,9 +2253,19 @@ template<class Ch, class Tr, class... T> struct ostream_insert_L
     }
 };
 
+template<class Os, class T, class E = void> struct is_output_streamable: std::false_type
+{
+};
+
+template<class Os, class T> struct is_output_streamable<Os, T, decltype( std::declval<Os&>() << std::declval<T const&>(), (void)0 )>: std::true_type
+{
+};
+
 } // namespace detail
 
-template<class Ch, class Tr, class T1, class... T> std::basic_ostream<Ch, Tr>& operator<<( std::basic_ostream<Ch, Tr>& os, variant<T1, T...> const& v )
+template<class Ch, class Tr, class T1, class... T,
+    class E = typename std::enable_if< mp11::mp_all< detail::is_output_streamable<std::basic_ostream<Ch, Tr>, T>... >::value >::type >
+std::basic_ostream<Ch, Tr>& operator<<( std::basic_ostream<Ch, Tr>& os, variant<T1, T...> const& v )
 {
     return mp11::mp_with_index<1 + sizeof...(T)>( v.index(),
         detail::ostream_insert_L<Ch, Tr, T1, T...>{ os, v } );
diff --git a/test/Jamfile b/test/Jamfile
index 0cc5dd2..2fbb975 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -125,3 +125,4 @@ compile variant_derived_construct.cpp ;
 run variant_visit_by_index.cpp ;
 
 run variant_ostream_insert.cpp ;
+run is_output_streamable.cpp ;
diff --git a/test/is_output_streamable.cpp b/test/is_output_streamable.cpp
new file mode 100644
index 0000000..6d0f0a2
--- /dev/null
+++ b/test/is_output_streamable.cpp
@@ -0,0 +1,38 @@
+// Copyright 2021 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 <boost/core/lightweight_test_trait.hpp>
+#include <ostream>
+#include <string>
+
+struct X
+{
+};
+
+struct Y
+{
+};
+
+std::ostream& operator<<( std::ostream& os, Y const& /*y*/ )
+{
+    os << "Y()";
+    return os;
+}
+
+int main()
+{
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, int>));
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, float>));
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, std::string>));
+    BOOST_TEST_TRAIT_FALSE((boost::variant2::detail::is_output_streamable<std::ostream, void>));
+    BOOST_TEST_TRAIT_FALSE((boost::variant2::detail::is_output_streamable<std::ostream, X>));
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, Y>));
+
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, boost::variant2::variant<int, float, std::string>>));
+    BOOST_TEST_TRAIT_FALSE((boost::variant2::detail::is_output_streamable<std::ostream, boost::variant2::variant<int, float, X>>));
+    BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, boost::variant2::variant<int, float, Y>>));
+
+    return boost::report_errors();
+}