[gcc r10-9570] libstdc++: Fix divide by zero in default template argument

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:8ee9e0fac5840278d0ff03c465be643959216904

commit r10-9570-g8ee9e0fac5840278d0ff03c465be643959216904
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Oct 8 00:34:56 2020 +0100

    libstdc++: Fix divide by zero in default template argument
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/random.h (__detail::_Mod): Avoid divide by zero.
            * testsuite/26_numerics/random/linear_congruential_engine/operators/call.cc:
            New test.
    
    (cherry picked from commit 6ae17a3b6835b30102607d45ac89c7a668e2c8d4)

Diff:
---
 libstdc++-v3/include/bits/random.h                 |  2 +-
 .../linear_congruential_engine/operators/call.cc   | 27 ++++++++++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/random.h b/libstdc++-v3/include/bits/random.h
index 4a6558c966a..920f3d91513 100644
--- a/libstdc++-v3/include/bits/random.h
+++ b/libstdc++-v3/include/bits/random.h
@@ -109,7 +109,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     template<typename _Tp, _Tp __m, _Tp __a, _Tp __c,
 	     bool __big_enough = (!(__m & (__m - 1))
 				  || (_Tp(-1) - __c) / __a >= __m - 1),
-             bool __schrage_ok = __m % __a < __m / __a>
+             bool __schrage_ok = __a != 0 && __m % __a < __m / __a>
       struct _Mod
       {
 	typedef typename _Select_uint_least_t<std::__lg(__a)
diff --git a/libstdc++-v3/testsuite/26_numerics/random/linear_congruential_engine/operators/call.cc b/libstdc++-v3/testsuite/26_numerics/random/linear_congruential_engine/operators/call.cc
new file mode 100644
index 00000000000..d1fff6d0a5d
--- /dev/null
+++ b/libstdc++-v3/testsuite/26_numerics/random/linear_congruential_engine/operators/call.cc
@@ -0,0 +1,27 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-do compile { target c++11 } }
+
+#include <random>
+
+unsigned
+test01()
+{
+  std::linear_congruential_engine<unsigned, 0, 0, 0> l;
+  return l(); // this used to result in divide by zero
+}