fromJson()/toJson() refactoring

fromJson() is generalised to accept any JSON-like type while passing
QJsonObject to JsonConverter<>::load (instead of doLoad). This allows to
(still) rely on JsonConverter<> as a customisation point while providing
an opportunity to overload fromJson for custom types in a pointed way
(specifically, by providing the overload for
`fromJson(const QJsonObject&)`), instead of having to go with full-blown
JsonConverter<> specialisation. This will be used in a further commit
to simplify ReceiptEvent definition.

Using if constexpr in combination with constraints (`requires()`) -
the first such case in Quotient codebase - allowed to put the entire
logic in a single JsonConverter<>::load() body instead of having a
facility JsonExporter<> class for SFINAE.

Aside from that, fromJson<QJsonValue, QJsonValue> is entirely dropped
because it's not supposed to be used that way (it's no-op after all);
reflecting that, Event::unsignedPart() and Event::contentPart() no more
default to QJsonValue as the expected return type, you have to
explicitly provide the type instead (and as one can see from the changes
in the commit, it's actually better that way since it's better
to validate the value inside JSON - e.g. check QString or QJsonObject
for emptiness - than the QJsonValue envelope which may still wrap
an empty value).

toJson() is also generalised, replacing 3 functions with one that has
a constexpr if to discern between two kinds of types.

4 files changed, 12 insertions, 10 deletions

diff --git a/lib/events/encryptedevent.cpp b/lib/events/encryptedevent.cpp
index c97ccc16..ec00ad4c 100644
--- a/lib/events/encryptedevent.cpp
+++ b/lib/events/encryptedevent.cpp
@@ -49,14 +49,16 @@ RoomEventPtr EncryptedEvent::createDecrypted(const QString &decrypted) const
     eventObject["event_id"] = id();
     eventObject["sender"] = senderId();
     eventObject["origin_server_ts"] = originTimestamp().toMSecsSinceEpoch();
-    if (const auto relatesToJson = contentPart("m.relates_to"_ls); !relatesToJson.isUndefined()) {
+    if (const auto relatesToJson = contentPart<QJsonObject>("m.relates_to"_ls);
+        !relatesToJson.isEmpty()) {
         auto content = eventObject["content"].toObject();
-        content["m.relates_to"] = relatesToJson.toObject();
+        content["m.relates_to"] = relatesToJson;
         eventObject["content"] = content;
     }
-    if (const auto redactsJson = unsignedPart("redacts"_ls); !redactsJson.isUndefined()) {
+    if (const auto redactsJson = unsignedPart<QString>("redacts"_ls);
+        !redactsJson.isEmpty()) {
         auto unsign = eventObject["unsigned"].toObject();
-        unsign["redacts"] = redactsJson.toString();
+        unsign["redacts"] = redactsJson;
         eventObject["unsigned"] = unsign;
     }
     return loadEvent<RoomEvent>(eventObject);
diff --git a/lib/events/event.h b/lib/events/event.h
index 05eb51e9..da6cf3c7 100644
--- a/lib/events/event.h
+++ b/lib/events/event.h
@@ -212,7 +212,7 @@ public:
 
     const QJsonObject contentJson() const;
 
-    template <typename T = QJsonValue, typename KeyT>
+    template <typename T, typename KeyT>
     const T contentPart(KeyT&& key) const
     {
         return fromJson<T>(contentJson()[std::forward<KeyT>(key)]);
@@ -227,7 +227,7 @@ public:
 
     const QJsonObject unsignedJson() const;
 
-    template <typename T = QJsonValue, typename KeyT>
+    template <typename T, typename KeyT>
     const T unsignedPart(KeyT&& key) const
     {
         return fromJson<T>(unsignedJson()[std::forward<KeyT>(key)]);
diff --git a/lib/events/roomevent.cpp b/lib/events/roomevent.cpp
index 3ddf5ac4..e695e0ec 100644
--- a/lib/events/roomevent.cpp
+++ b/lib/events/roomevent.cpp
@@ -15,9 +15,9 @@ RoomEvent::RoomEvent(Type type, event_mtype_t matrixType,
 
 RoomEvent::RoomEvent(Type type, const QJsonObject& json) : Event(type, json)
 {
-    if (const auto redaction = unsignedPart(RedactedCauseKeyL);
-        redaction.isObject())
-        _redactedBecause = makeEvent<RedactionEvent>(redaction.toObject());
+    if (const auto redaction = unsignedPart<QJsonObject>(RedactedCauseKeyL);
+        !redaction.isEmpty())
+        _redactedBecause = makeEvent<RedactionEvent>(redaction);
 }
 
 RoomEvent::~RoomEvent() = default; // Let the smart pointer do its job
diff --git a/lib/events/stateevent.cpp b/lib/events/stateevent.cpp
index c343e37f..43dfd6e8 100644
--- a/lib/events/stateevent.cpp
+++ b/lib/events/stateevent.cpp
@@ -21,7 +21,7 @@ StateEventBase::StateEventBase(Event::Type type, event_mtype_t matrixType,
 
 bool StateEventBase::repeatsState() const
 {
-    const auto prevContentJson = unsignedPart(PrevContentKeyL);
+    const auto prevContentJson = unsignedPart<QJsonObject>(PrevContentKeyL);
     return fullJson().value(ContentKeyL) == prevContentJson;
 }