[Bug libstdc++/71434] New: binomial_distribution operator(): using uninitialized variable

["barannikov88 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71434

            Bug ID: 71434
           Summary: binomial_distribution operator(): using uninitialized
                    variable
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barannikov88 at gmail dot com
  Target Milestone: ---

The variable __x declared at line 1699 of file
libstdc++-v3/include/bits/random.tcc may be used uninitialized. This happens
when the following conditions are met:
__u <= __a1 (line 1724)
__y >= __param._M_d1 (line 1728)

When both conditions are TRUE, the variable __x is used uninitialized at line
1782:

__reject |= __x + __np >= __thr;

Seems that it is assumed that __x has a meaningful value only when __reject ==
FALSE (other uses of __x are under this condition). If __reject is TRUE, then
the value of the variable __x doesn't matter: __reject will remain TRUE. But
__x is used unconditionally it this context, that leads to execution error on a
machine with tagged architecture (this is how I found it).

This line should be rewritten as:
__reject = __reject || __x + __np >= __thr;

(note that if __reject is TRUE, the variable __x is not used.)

This applies to all versions of libstdc++. Line numbers provided are for
gcc-5.1.