Comparisons with concrete types break in the presence of inapplicable where bounds

[robertbastian]

See https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=f6449d0fee6e0875b3f92612a5be84bd

[compiler-errors]

This is probably due to SelectionContext::candidate_should_be_dropped_in_favor_of preferring param-env candidates (in the where-clause) over impl candidates.

The reason that &1u8 == &2u8 works in the first function's first line is that this impl candidate applies first when we have &_ compared to &_ and doesn't overlap with the param-env candidate.

[compiler-errors]

Not exactly sure how to fix this without either:

1. adjusting the logic here, but I would expect that to have fallout in many other places
2. doing some hacks to infer the real types of the generic arguments we pass to, e.g., <_ as Add<_>>::add, but I fear that would mess with coercions that are used to, e.g., let us do String + &String even without an impl that matches this signature exactly (i.e. coercing into this impl)

[fmease]

Closing as duplicate of #24066 similar to #84218.