IRGen: Terrible workaround for problem in searchNominalTypeMetadata() #85357
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This fixes a regression introduced by e3c8f42, but the root cause was actually a subtle invariant violation in IRGen. FulfillmentMap's use of canonical types as keys assumes that canonical type equality is sufficient for checking if two types "are the same". However, this is not true when those types contain type parameters, because two distinct type parameters might belong to the same equivalence class. Thus, when we take the type's context substitution map, and apply it to each type parameter in our given list of requirements, the substitution operation could output a different but equivalent type parameter. As a workaround for this problem, try to preserve the old behavior of subst() in this specific case. Fixes rdar://160649141.
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This fixes a regression introduced by e3c8f42, but the root cause was actually a subtle invariant violation in IRGen.
FulfillmentMap's use of canonical types as keys assumes that canonical type equality is sufficient for checking if two types "are the same". However, this is not true when those types contain type parameters, because two distinct type parameters might belong to the same equivalence class.
Thus, when we take the type's context substitution map, and apply it to each type parameter in our given list of requirements, the substitution operation could output a different but equivalent type parameter.
As a workaround for this problem, try to preserve the old behavior of subst() in this specific case.
Fixes rdar://160649141.