PL312 25 [algorithms] Fix constraints on ranges comparison algorithms P1716 #308
Description
The ranges::is_permutation, ranges::unique, ranges::unique_copy algorithm are currently underconstrained, as they do not require supplied functor to model equivalence relation.
Other compare algorithms (like ranges::equal, ranges::mismatch, ranges::search), are overconstrained. They require supplied functor to model relation<T, U> concept instead of predicate<T, U>, where T and U are reference types of supplied ranges. As consequence, supplied functor needs to be callable with four combinations of arguments: (T, U), (U, T), (U, U), (U, U), instead of just (T, U). This makes them less general than existing non-range overloads, and complicates code migration.
Proposed change:
Adopt P1716R2.