Fix trait upcasting to dyn type with no principal when there are projections #139421
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#126660 (which I had originally authored, lol) had a subtle bug that is the moral equivalent of #114036, which is that when upcasting from
dyn Principal<Projection = Ty> + AutoTraittodyn AutoTrait, we were dropping the trait ref forPrincipalbut not its projections (if there were any).With debug assertions enabled, this triggers the assertion I luckily added in a2a0cfe, but even without debug assertions this is a logical bug since we had a dyn type with just a projection bound but no principal, so it caused a type mismatch.
This does not need an FCP because this should've been covered by the FCP in #126660, but we just weren't testing a case when casting from a
dyntype with projections 😸Fixes #139418
r? @oli-obk (or anyone)