Canon Supertelephoto Prime with Built-In Teleconverter Confirmed

Canon is working on a supertelephoto prime with a built-in teleconverter, per Canon Rumors, with an unconfirmed spec of RF 400mm f/2.8L IS with a 1.5x converter built in. That would give 600mm at f/4.2 in the same barrel. Canon Rumors says the design will differ from current built-in teleconverter implementations, without specifying how. RF-native versions of the 400mm f/2.8L and 600mm f/4L have been delayed, with parts shortages cited.