Fix conversion to std::optional (#5052)

* 🐛 fix conversion to std::optional

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 🚨 fix warning

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 🚨 fix warning

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

Author: nlohmann

include/nlohmann/detail/conversions/from_json.hpp

@@ -53,7 +53,8 @@ inline void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
 }
 
 #ifdef JSON_HAS_CPP_17
-template<typename BasicJsonType, typename T>
+template < typename BasicJsonType, typename T,
+           typename std::enable_if < !nlohmann::detail::is_basic_json<T>::value, int >::type = 0 >
 void from_json(const BasicJsonType& j, std::optional<T>& opt)
 {
     if (j.is_null())

single_include/nlohmann/json.hpp

@@ -4889,7 +4889,8 @@ inline void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
 }
 
 #ifdef JSON_HAS_CPP_17
-template<typename BasicJsonType, typename T>
+template < typename BasicJsonType, typename T,
+           typename std::enable_if < !nlohmann::detail::is_basic_json<T>::value, int >::type = 0 >
 void from_json(const BasicJsonType& j, std::optional<T>& opt)
 {
     if (j.is_null())

tests/src/unit-regression2.cpp

@@ -1118,6 +1118,21 @@ TEST_CASE("regression tests 2")
         const auto decoded = json_4804::from_cbor(data);
         CHECK((decoded == json_4804::array()));
     }
+
+    SECTION("issue #5046 - implicit conversion of return json to std::optional no longer implicit")
+    {
+        const json jval{};
+        auto GetValue = [](const json & valRoot) -> std::optional<json>
+        {
+            if (valRoot.contains("default"))
+            {
+                return valRoot.at("default");
+            }
+            return std::nullopt;
+        };
+        auto result = GetValue(jval);
+        CHECK(!result.has_value());
+    }
 #endif
 }