The operator*() for QJsonArray::const_iterator and QJsonObject::const_iterator is defined as follows:

inline const QJsonValueRef operator*() const { return item; }

Since the const qualifier on the return type by value is irrelevant, this means we can modify a JSON array or object even when it is const. This compiles without warning and modifies the array:

const QJsonArray array = ...;
for (auto v : array)
    v = 0;

I'm not sure how this could be fixed. In Qt 5, this operator*() returned a QJsonValue, so the potential modification was limited to the value v. Of course the fix in the user code could be to write for (const auto v : array) instead.