[committed] libstdc++: Add assertions for preconditions in sampling distributions [PR 82584]

[Jonathan Wakely via Gcc-patches]

These three distributions all require 0 < S where S is the sum of the
weights. When the sum is zero there's an undefined FP division by zero.
Add assertions to help users diagnose the problem.

libstdc++-v3/ChangeLog:

        PR libstdc++/82584
        * include/bits/random.tcc
        (discrete_distribution::param_type::_M_initialize)
        (piecewise_constant_distribution::param_type::_M_initialize)
        (piecewise_linear_distribution::param_type::_M_initialize):
        Add assertions for positive sums..
        * testsuite/26_numerics/random/pr60037-neg.cc: Adjust dg-error
        line.

Tested powerpc64le-linux. Committed to trunk.


commit b2a96bf9dce41ee777b1669e4a8b4c6df3ff3613
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Thu Oct 8 15:14:58 2020

    libstdc++: Add assertions for preconditions in sampling distributions [PR 
82584]
    
    These three distributions all require 0 < S where S is the sum of the
    weights. When the sum is zero there's an undefined FP division by zero.
    Add assertions to help users diagnose the problem.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/82584
            * include/bits/random.tcc
            (discrete_distribution::param_type::_M_initialize)
            (piecewise_constant_distribution::param_type::_M_initialize)
            (piecewise_linear_distribution::param_type::_M_initialize):
            Add assertions for positive sums..
            * testsuite/26_numerics/random/pr60037-neg.cc: Adjust dg-error
            line.

diff --git a/libstdc++-v3/include/bits/random.tcc 
b/libstdc++-v3/include/bits/random.tcc
index f67b2fe48f4..a921b9bf815 100644
--- a/libstdc++-v3/include/bits/random.tcc
+++ b/libstdc++-v3/include/bits/random.tcc
@@ -2630,6 +2630,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       const double __sum = std::accumulate(_M_prob.begin(),
                                           _M_prob.end(), 0.0);
+      __glibcxx_assert(__sum > 0);
       // Now normalize the probabilites.
       __detail::__normalize(_M_prob.begin(), _M_prob.end(), _M_prob.begin(),
                            __sum);
@@ -2794,6 +2795,7 @@ namespace __detail
 
       const double __sum = std::accumulate(_M_den.begin(),
                                           _M_den.end(), 0.0);
+      __glibcxx_assert(__sum > 0);
 
       __detail::__normalize(_M_den.begin(), _M_den.end(), _M_den.begin(),
                            __sum);
@@ -3016,6 +3018,7 @@ namespace __detail
          _M_cp.push_back(__sum);
          _M_m.push_back((_M_den[__k + 1] - _M_den[__k]) / __delta);
        }
+      __glibcxx_assert(__sum > 0);
 
       //  Now normalize the densities...
       __detail::__normalize(_M_den.begin(), _M_den.end(), _M_den.begin(),
diff --git a/libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc 
b/libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc
index 139abbb3051..9cffc3d06f9 100644
--- a/libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc
+++ b/libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc
@@ -12,4 +12,4 @@ auto x = std::generate_canonical<std::size_t,
 
 // { dg-error "static assertion failed: template argument must be a floating 
point type" "" { target *-*-* } 167 }
 
-// { dg-error "static assertion failed: template argument must be a floating 
point type" "" { target *-*-* } 3281 }
+// { dg-error "static assertion failed: template argument must be a floating 
point type" "" { target *-*-* } 3284 }