instance: only polymorphize upvar substs #75337
Merged
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This PR restricts the substitution polymorphization added in #75255 to only apply to the tupled upvar substitution, rather than all substitutions, fixing a bunch of regressions when polymorphization is
enabled.
Due to an oversight on my part, when landing #75260 and #75255, some tests started failing when polymorphization was enabled that I didn't notice until after landing - this PR fixes the regressions from #75255. #75336 has been filed to make sure that we don't forget to try make this change again in future, as it does enable some optimisations.
r? @lcnr