Adding the following code is a workaround:

namespace nlohmann
{
template
struct adl_serializer<std::optional>
{
static void to_json(json& j, std::optional const& o)
{
if (!o) { j = nullptr; } else { j = *o; }
}
static std::optional from_json(json const& j)
{
return j.is_null() ? std::make_optional() : std::make_optional(j.get());
}
};
};

Problem with #ifndef JSON_USE_IMPLICIT_CONVERSIONS in from_json.hpp I think.
The test should be #if JSON_USE_IMPLICIT_CONVERSIONS==0 or something like that,, or remove the #ifndef completely.
When not defined it's forced to 1 in macro_scope.hpp and if defined to 0 then the #ifndef blocks the insertion of code.

Good catch!

However, after replacing the #ifdef with an #if and adding a test like this:

const auto t0 = Example_4740();
const auto j = nlohmann::json(t0);
CHECK(j.dump() == "{\"host\":null,\"port\":null}");

all is fine, but adding

auto t1 = j.get<Example_4740>();
CHECK(!t1.host.has_value());
CHECK(!t1.port.has_value());

again breaks the build.

Now, I am unsure why we have the check for JSON_USE_IMPLICIT_CONVERSIONS in the first place.

I created a PR #4742 to test this. Please take a look.

I think the JSON_USE_IMPLICIT_CONVERSIONS check should be removed. The test code should compile and work with JSON_USE_IMPLICIT_CONVERSIONS set to 0 or set to 1.

Thanks, it does - the MR has a green CI and I only need to add another test to improve the code coverage.

@vtstmn Please take a look at #4742.