From b2a96bf9dce41ee777b1669e4a8b4c6df3ff3613 Mon Sep 17 00:00:00 2001
From: Jonathan Wakely <jwakely@redhat.com>
Date: Thu, 8 Oct 2020 15:14:58 +0100
Subject: [PATCH] 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.
---
 libstdc++-v3/include/bits/random.tcc                     | 3 +++
 libstdc++-v3/testsuite/26_numerics/random/pr60037-neg.cc | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/random.tcc b/libstdc++-v3/include/bits/random.tcc
index f67b2fe..a921b9b 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 139abbb..9cffc3d 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 }
-- 
2.7.4