This should not need much of additional markup in the .serializers.in file.
The fix should be to use different pattern to decode

std::optional<Ref<WebCore::TimingFunction>> ArgumentCoder<WebCore::TimingFunction>::decode(Decoder& decoder)
{
    std::optional<WebCore_TimingFunction_Subclass> type;
    decoder >> type;
    if (!type)
        return std::nullopt;

    if (type == WebCore_TimingFunction_Subclass::LinearTimingFunction) {
        std::optional<Ref<WebCore::LinearTimingFunction>> result;
        decoder >> result;
        if (!result)
            return std::nullopt;
        return WTFMove(*result);
    }


should be

std::optional<Ref<WebCore::TimingFunction>> ArgumentCoder<WebCore::TimingFunction>::decode(Decoder& decoder)
{
    auto type = decoder.decode<WebCore_TimingFunction_Subclass>();
    if (UNLIKELY(!decoder.isValid()))
        return std::nullopt;

    if (type == WebCore_TimingFunction_Subclass::LinearTimingFunction) {
        auto result = decoder.decode<Ref<WebCore::LinearTimingFunction>>()
        if (UNLIKELY(!decoder.isValid()))
            return std::nullopt;
        return WTFMove(*result);
    }

Pull request: https://github.com/WebKit/WebKit/pull/14658

Committed 264912@main (6b8b65111faf): <https://commits.webkit.org/264912@main>

Reviewed commits have been landed. Closing PR #14658 and removing active labels.

<rdar://problem/110350309>