Add per-ctor verify() functions that take care of the warnings for invalid inputs, and return a struct { QObject*; QMetaMethod; } to feed into a delegatee constructor. This solves the problem that the varying parts between the ctors are at the beginning, not the end, using another level of indirection, and will eventually allow to make the `args` and `sig` members const, and therefore remove the need to protect either with the mutex. This patch tries to keep the diff minimal. I'm planning to de-inline most of the class in a future commit, so it doesn't matter that private and public sections are interleaved, that will be cleaned up when the code is moved to a .cpp file later. Task-number: QTBUG-123544 Change-Id: Idc628c927736880a8fd580089ed5177361c94ed9 Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>