We have code attempting to do conversions in  ExecutionEngine::toVariant, ExecutionEngine::metaTypeFromJS, QQmlPropertyPrivate::write and also in the QQmlPropertyValidator.

This is the cause of annoying inconsistencies, duplicated (but slightly different) code paths and generally terrible for maintenance.

While there are different use cases which warrant different behaviour to some extent, we should unify this as much as possible (and then also document and exhaustively test it).