Abort when ggproto definition is circular

[teunbrand]

This PR aims to fix #6583.

AFAICT, we cannot detect this during object construction.
The second best way to detect this is in fetch_ggproto(), as it is such a widely used accessor.
Reprex from issue:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

A <- ggproto("A")
A <- ggproto(NULL, A)
A
#> Error in `fetch_ggproto()` at ggplot2/R/ggproto.R:172:3:
#> ! <A> cannot have a circular definition.

^{Created on 2025-09-25 with reprex v2.1.1}

[thomasp85]

Does this catch circular inheritance that has a larger radius, e.g. A->B->C->A?

[teunbrand]

Good question. No it does not. I imagine this is a less common occurrance of an already obscure problem. I don't have instant inspiration on how this would be solved without laboriously tracking all parents, which may not be worth it for throwing a rare error message.

[thomasp85]

I don't think I've ever stumbled upon this issue in any form so I can't be the judge of that. If you feel this PR solves a real issue and what I brought up is contrived, then I'm fine accepting as-is

[teunbrand]

Ok, if Thomas has never encountered this issue, I'm going to argue that this too obscure of an error to be worth fixing. I'll close this PR andn the related issue.