simplify exception wrapping in value_to

include/boost/json/detail/value_to.hpp

@@ -525,55 +525,6 @@ initialize_variant( V&& v, mp11::mp_int<1> )
 }
 #endif // BOOST_NO_CXX17_HDR_VARIANT
 
-struct locally_prohibit_exceptions
-{};
-
-template< class Ctx >
-Ctx const&
-make_locally_nonthrowing_context(Ctx const& ctx) noexcept
-{
-    return ctx;
-}
-
-template< class... Ctxes >
-std::tuple<Ctxes...> const&
-make_locally_nonthrowing_context(std::tuple<Ctxes...> const& ctx) noexcept
-{
-    return ctx;
-}
-
-template< class... Ctxes >
-std::tuple<locally_prohibit_exceptions, allow_exceptions, Ctxes...>
-make_locally_nonthrowing_context(std::tuple<allow_exceptions, Ctxes...> const& ctx)
-    noexcept
-{
-    return std::tuple_cat(std::make_tuple( locally_prohibit_exceptions() ), ctx);
-}
-
-template< class Ctx >
-Ctx const&
-remove_local_exception_prohibition(Ctx const& ctx) noexcept
-{
-    return ctx;
-}
-
-template< class T, class... Ts, std::size_t... Is>
-std::tuple<Ts...>
-remove_local_exception_prohibition_helper(
-    std::tuple<T, Ts...> const& tup,
-    mp11::index_sequence<Is...>) noexcept
-{
-    return std::tuple<Ts...>( std::get<Is + 1>(tup)... );
-}
-
-template< class... Ctxes >
-std::tuple<Ctxes...>
-remove_local_exception_prohibition(
-    std::tuple<locally_prohibit_exceptions, Ctxes...> const& ctx) noexcept
-{
-    return remove_local_exception_prohibition_helper(
-        ctx, mp11::index_sequence_for<Ctxes...>() );
-}
 
 template< class T, class Ctx >
 struct alternative_converter
@@ -588,9 +539,8 @@ struct alternative_converter
         if( res )
             return;
 
-        auto&& local_ctx = make_locally_nonthrowing_context(ctx);
         using V = mp11::mp_at<T, I>;
-        auto attempt = try_value_to<V>(jv, local_ctx);
+        auto attempt = try_value_to<V>(jv, ctx);
         if( attempt )
         {
             using cat = variant_construction_category<T, V, I>;
@@ -709,28 +659,6 @@ value_to_impl(
     return std::move(*res);
 }
 
-template< class Ctx >
-std::tuple<allow_exceptions, Ctx>
-make_throwing_context(Ctx const& ctx)
-{
-    return std::tuple<allow_exceptions, Ctx>(allow_exceptions(), ctx);
-}
-
-template< class... Ctxes >
-std::tuple<allow_exceptions, Ctxes...>
-make_throwing_context(std::tuple<Ctxes...> const& ctx)
-{
-    return std::tuple_cat(std::make_tuple( allow_exceptions() ), ctx);
-}
-
-template< class... Ctxes >
-std::tuple<allow_exceptions, Ctxes...> const&
-make_throwing_context(std::tuple<allow_exceptions, Ctxes...> const& ctx)
-    noexcept
-{
-    return ctx;
-}
-
 template<
     class T,
     class Ctx,
@@ -746,11 +674,7 @@ value_to_impl(
     value const& jv,
     Ctx const& ctx )
 {
-    auto res = tag_invoke(
-        try_value_to_tag<T>(),
-        jv,
-        Sup::get(ctx),
-        make_throwing_context(ctx));
+    auto res = tag_invoke(try_value_to_tag<T>(), jv, Sup::get(ctx), ctx);
     if( res.has_error() )
         throw_system_error( res.error() );
     return std::move(*res);
@@ -809,36 +733,17 @@ value_to_impl(
 //----------------------------------------------------------
 // User-provided conversions; nonthrowing -> throwing
 
-template< class Ctx >
-struct does_allow_exceptions : std::false_type
-{ };
-
-template< class... Ctxes >
-struct does_allow_exceptions< std::tuple<allow_exceptions, Ctxes...> >
-    : std::true_type
-{ };
-
-template< class T, class... Args >
-system::result<T>
-wrap_conversion_exceptions( std::true_type, value_to_tag<T>, Args&& ... args )
-{
-    return {
-        boost::system::in_place_value,
-        tag_invoke( value_to_tag<T>(), static_cast<Args&&>(args)... )};
-}
-
 template< class T, class... Args >
 system::result<T>
-wrap_conversion_exceptions( std::false_type, value_to_tag<T>, Args&& ... args )
+wrap_conversion_exceptions( value_to_tag<T>, Args&& ... args )
 {
 #ifndef BOOST_NO_EXCEPTIONS
     try
     {
 #endif
-        return wrap_conversion_exceptions(
-            std::true_type(),
-            value_to_tag<T>(),
-            static_cast<Args&&>(args)... );
+        return {
+            boost::system::in_place_value,
+            tag_invoke( value_to_tag<T>(), static_cast<Args&&>(args)... )};
 #ifndef BOOST_NO_EXCEPTIONS
     }
     catch( std::bad_alloc const&)
@@ -865,8 +770,7 @@ mp11::mp_if_c<
 value_to_impl(
     user_conversion_tag, try_value_to_tag<T>, value const& jv, Ctx const& )
 {
-    return wrap_conversion_exceptions(
-        does_allow_exceptions<Ctx>(), value_to_tag<T>(), jv);
+    return wrap_conversion_exceptions(value_to_tag<T>(), jv);
 }
 
 template<
@@ -886,8 +790,7 @@ value_to_impl(
     value const& jv,
     Ctx const& ctx )
 {
-    return wrap_conversion_exceptions(
-        does_allow_exceptions<Ctx>(), value_to_tag<T>(), jv, Sup::get(ctx) );
+    return wrap_conversion_exceptions( value_to_tag<T>(), jv, Sup::get(ctx) );
 }
 
 template<
@@ -908,11 +811,7 @@ value_to_impl(
     Ctx const& ctx )
 {
     return wrap_conversion_exceptions(
-        does_allow_exceptions<Ctx>(),
-        value_to_tag<T>(),
-        jv,
-        Sup::get(ctx),
-        remove_local_exception_prohibition(ctx) );
+        value_to_tag<T>(), jv, Sup::get(ctx), ctx);
 }
 
 // no suitable conversion implementation
@@ -930,8 +829,7 @@ template< class Impl, class T, class Ctx >
 T
 value_to_impl( Impl impl, value_to_tag<T>, value const& jv, Ctx const& ctx )
 {
-    return value_to_impl(
-        impl, try_value_to_tag<T>(), jv, make_throwing_context(ctx) ).value();
+    return value_to_impl(impl, try_value_to_tag<T>(), jv, ctx).value();
 }
 
 template< class Ctx, class T >

include/boost/json/impl/conversion.hpp

@@ -387,9 +387,6 @@ struct conversion_category_impl< std::tuple<Ctxs...>, T, Dir >
 struct no_context
 {};
 
-struct allow_exceptions
-{};
-
 template <class T, class Dir>
 using can_convert = mp11::mp_not<
     std::is_same<

test/value_to.cpp

@@ -23,7 +23,6 @@
 #include <map>
 #include <unordered_map>
 #include <vector>
-#include <iostream>
 
 #ifndef BOOST_NO_CXX17_HDR_VARIANT
 # include <variant>
@@ -485,10 +484,9 @@ class value_to_test
 #endif // BOOST_NO_CXX17_HDR_OPTIONAL
         }
 
-        BOOST_TEST_THROWS(
+        BOOST_TEST_THROWS_WITH_LOCATION(
             value_to<::value_to_test_ns::T10>(
-                value{{"n", 0}, {"t3", "t10"}}, ctx... ),
-            std::invalid_argument);
+                value{{"n", 0}, {"t3", "t10"}}, ctx... ));
 #endif // BOOST_DESCRIBE_CXX14
     }