Diagnostics code produces malformed obligations

[lcnr]

#152661 papers over a larger more involved issue and we should fix the underlying issue here.

Yeah, it's not the next-solver that produces fewer args.
The problem is that obligation, i.e. its trait def just has the fewer args(=1) here and > it is ZeroablePrimitive in this case.
I'll look into why we are trying to report a fulfillment error on it.

Originally posted by @ShoyuVanilla in #152661 (comment)