Bug 18280 – std.algorithm.comparison.cmp for non-strings should call opCmp only once per item pair

Right now when comparing non-string ranges std.algorithm.comparison.cmp calls the comparison predicate twice for every pair of elements to determine their order. When the elements have overloaded `opCmp` and the predicate is "a < b" we only need to call `opCmp` once for each pair.

Commits pushed to master at https://github.com/dlang/phobos https://github.com/dlang/phobos/commit/7f59e5ad526ee4bf856e8235e3042bb5cd442ad4 Fix Issue 18280 - std.algorithm.comparison.cmp for non-strings should call opCmp only once per item pair split cmp into two overloads per @andralex https://github.com/dlang/phobos/pull/6056#pullrequestreview-90687092 Minor adjustments, again cmp should return auto and let opCmp drive https://github.com/dlang/phobos/pull/6056#issuecomment-359665184 Fix Issue 18285 - std.algorithm.comparison.cmp for strings with custom predicate compares lengths wrong Test std.algorithm.comparison.cmp when opCmp returns float Promotions should not use cast Optimize cmp's endgame There are some redundant tests when the end of the ranges is reached. Eliminated that, and improved threeWayByPred. Fix Issue 18286 - std.algorithm.comparison.cmp for string with custom predicate fails if distinct chars can compare equal Fix Issue 18288 - std.algorithm.comparison.cmp for wide strings should be @safe re-apply remove cast in promotions https://github.com/dlang/phobos/commit/2174695151633e64c33ef8c73dcd98ae2c27606e Merge pull request #6056 from n8sh/algorithm-cmp Fix Issue 18280 - std.algorithm.comparison.cmp for non-strings should call opCmp only once per item pair