[Issue 21810] New: Check for sortedness in `merge` must not use `==`

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21810

          Issue ID: 21810
           Summary: Check for sortedness in `merge` must not use `==`
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: minor
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: arne.ludwig at posteo.de

Checking sortedness according to a given predicate must not use `==` operator
because it may not be in accordance with the predicate.

In `std.algorithm.sorting.merge` this is happening which produces an
unjustified error in some cases.

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