https://issues.dlang.org/show_bug.cgi?id=21249

          Issue ID: 21249
           Summary: clamp() is not stable and is not constrained
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody@puremagic.com
          Reporter: andrei@erdani.com

For elaborate objects defining comparison, clamp() should return the leftmost object unless it falls outside the limits established by the other two parameters.

Consider:

    struct A {
        int x, y;
        int opCmp(ref const A rhs) const { return (x > rhs.x) - (x < rhs.x); }
    }
    A x, lo, hi;
    x.y = 42;
    assert(x.clamp(lo, hi).y == 42);

Also, clamp() needs to be constrained appropriately.

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