Fix converting a pair of pairs to JSON using json_auto templates (#1767)

Fixes: #1768
Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

Author: jviotti

src/core/json/include/sourcemeta/core/json_auto.h

@@ -3,11 +3,11 @@
 
 #include <sourcemeta/core/json_value.h>
 
-#include <concepts>    // std::same_as, std::constructible_from
-#include <functional>  // std::function
-#include <optional>    // std::optional
-#include <tuple>       // std::tuple, std::apply
-#include <type_traits> // std::false_type, std::true_type, std::void_t, std::is_enum_v, std::underlying_type_t, std::is_same_v
+#include <concepts>   // std::same_as, std::constructible_from
+#include <functional> // std::function
+#include <optional>   // std::optional
+#include <tuple> // std::tuple, std::apply, std::tuple_element_t, std::tuple_size, std::tuple_size_v
+#include <type_traits> // std::false_type, std::true_type, std::void_t, std::is_enum_v, std::underlying_type_t, std::is_same_v, std::is_base_of_v, std::remove_cvref_t
 #include <utility>     // std::pair
 
 namespace sourcemeta::core {
@@ -203,17 +203,37 @@ auto to_json(
 template <typename L, typename R>
 auto to_json(const std::pair<L, R> &value) -> JSON {
   auto tuple{JSON::make_array()};
-  tuple.push_back(to_json<L>(value.first));
-  tuple.push_back(to_json<R>(value.second));
+  tuple.push_back(to_json(value.first));
+  tuple.push_back(to_json(value.second));
   return tuple;
 }
 
+// Handle 1-element tuples
 /// @ingroup json
-template <typename... Args>
-auto to_json(const std::tuple<Args...> &value) -> JSON {
-  auto tuple{JSON::make_array()};
+template <typename T>
+  requires(std::tuple_size_v<std::remove_cvref_t<std::tuple<T>>> == 1)
+auto to_json(const std::tuple<T> &value) -> JSON {
+  auto tuple = JSON::make_array();
+  std::apply([&](const T &element) { tuple.push_back(to_json(element)); },
+             value);
+  return tuple;
+}
+
+// We have to do this mess because MSVC seems confuses `std::pair`
+// of 2 elements with this overload
+/// @ingroup json
+template <typename TupleT>
+  requires(requires {
+    typename std::tuple_size<std::remove_cvref_t<TupleT>>::type;
+  } && (std::tuple_size_v<std::remove_cvref_t<TupleT>> >= 2) &&
+           (!std::is_base_of_v<
+               std::pair<std::tuple_element_t<0, std::remove_cvref_t<TupleT>>,
+                         std::tuple_element_t<1, std::remove_cvref_t<TupleT>>>,
+               std::remove_cvref_t<TupleT>>))
+auto to_json(const TupleT &value) -> JSON {
+  auto tuple = JSON::make_array();
   std::apply(
-      [&tuple](const Args &...elements) {
+      [&tuple](const auto &...elements) {
         (tuple.push_back(to_json(elements)), ...);
       },
       value);

test/json/json_to_json_test.cc

@@ -173,6 +173,18 @@ TEST(JSON_to_json, pair) {
   EXPECT_EQ(result, expected);
 }
 
+TEST(JSON_to_json, pair_with_pair) {
+  const std::pair<sourcemeta::core::JSON::Type,
+                  std::pair<sourcemeta::core::JSON::Type, std::size_t>>
+      value{sourcemeta::core::JSON::Type::String,
+            {sourcemeta::core::JSON::Type::String, 5}};
+
+  const auto result{sourcemeta::core::to_json(value)};
+  const auto expected{
+      sourcemeta::core::parse_json(R"JSON([ 4, [ 4, 5 ] ])JSON")};
+  EXPECT_EQ(result, expected);
+}
+
 TEST(JSON_to_json, tuple_1) {
   const std::tuple<std::string> value{"foo"};
   const auto result{sourcemeta::core::to_json(value)};