[Bug libstdc++/80137] New: std::generate_canonical calls its generator a non-constant number of times

[john.salmon at deshaw dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80137

            Bug ID: 80137
           Summary: std::generate_canonical calls its generator a
                    non-constant number of times
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: john.salmon at deshaw dot com
  Target Milestone: ---

The fix to bug 63176 causes std::generate_canonical to loop
until the result is less than 1.0.  As noted in the
discussion, this breaks the complexity requirement in
[rand.util.canonical]/p3.

There is a simple solution which also fixes 63176 but
which doesn't violate the standard's complexity requirement.
Instead of looping, do:

   if (__builtin_expect(__ret >= _RealType(1), 0))
       return _RealType(std::nextafter(_RealType(1), _RealType(0)));
   else
       return _ret;