Describe the feature

I was building a building, and while building this building, I came across a problem while building the building. You see, I had a diagonal wall(To make it interesting, and I also wanted a sloped roof. Where the two met, I had a problem. There is no right triangular pyramid, which would allow you to merge these together.
Think of it like this image:

You could have a diagonal wall beneath, and could connect it to the rest of the roof, with out there being the part hanging off seen in this photo:

It would make building with slopes so much easier. Please consider this!

Unless I'm misunderstanding what you mean, this is already covered by the Framed Prism Corner and Framed Threeway Corner and their inner variants

Is this in 1.18.2? Are these planned blocks or have they already been added?

Those blocks exist since at least 1.16.5, maybe even since 1.15.2.

Just tried it out, the threeway doesn't work, but the prism corner does! Thank you very much!