PR77994 Convert std::sample size argument to suitable type

diff --git a/libstdc++-v3/include/bits/stl_algo.h b/libstdc++-v3/include/bits/stl_algo.h
index db99cb8..6c771bb 100644
--- a/libstdc++-v3/include/bits/stl_algo.h
+++ b/libstdc++-v3/include/bits/stl_algo.h
@@ -5730,8 +5730,9 @@  _GLIBCXX_BEGIN_NAMESPACE_ALGO
       static_assert(is_integral<_Distance>::value,
 		    "sample size must be an integer type");
 
+      typename iterator_traits<_PopulationIterator>::difference_type __d = __n;
       return std::__sample(__first, __last, __pop_cat{}, __out, __samp_cat{},
-			   __n, std::forward<_UniformRandomBitGenerator>(__g));
+			   __d, std::forward<_UniformRandomBitGenerator>(__g));
     }
 #endif // C++17
 #endif // C++14
diff --git a/libstdc++-v3/include/experimental/algorithm b/libstdc++-v3/include/experimental/algorithm
index eb18dde..ff66b43 100644
--- a/libstdc++-v3/include/experimental/algorithm
+++ b/libstdc++-v3/include/experimental/algorithm
@@ -76,8 +76,9 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static_assert(is_integral<_Distance>::value,
 		    "sample size must be an integer type");
 
+      typename iterator_traits<_PopulationIterator>::difference_type __d = __n;
       return std::__sample(__first, __last, __pop_cat{}, __out, __samp_cat{},
-			   __n,
+			   __d,
 			   std::forward<_UniformRandomNumberGenerator>(__g));
     }
 
diff --git a/libstdc++-v3/testsuite/25_algorithms/sample/2.cc b/libstdc++-v3/testsuite/25_algorithms/sample/2.cc
new file mode 100644
index 0000000..8865e18
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/sample/2.cc
@@ -0,0 +1,65 @@ 
+// Copyright (C) 2016 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-options "-std=gnu++17" }
+// { dg-do run { target c++1z } }
+
+#ifndef _GLIBCXX_ASSERTIONS
+// Make std::uniform_int_distribution check its parameters
+# define _GLIBCXX_ASSERTIONS
+#endif
+
+#include <algorithm>
+#include <random>
+#include <climits>
+#include <testsuite_hooks.h>
+#include <testsuite_iterators.h>
+
+std::mt19937 rng;
+
+using std::sample;
+using __gnu_test::test_container;
+using __gnu_test::output_iterator_wrapper;
+
+void
+test01()
+{
+  int pop[UCHAR_MAX]  = { };
+  for (int i = SCHAR_MAX; i < UCHAR_MAX; ++i)
+    pop[i] = 1;
+  const signed char sample_size = SCHAR_MAX; // PR libstdc++/77994
+  int out[sample_size] = { };
+
+  // random access iterators for both population and result
+  // (uses reservoir sampling)
+  auto it = sample(std::begin(pop), std::end(pop), out, sample_size, rng);
+  auto sum = std::accumulate(out, it, 0);
+  VERIFY( sum != 0 ); // exceedingly unlikely!
+
+  // random access iterator for population and output iterator for result
+  // (uses selection sampling)
+  test_container<int, output_iterator_wrapper> samp2(out);
+  sample(std::begin(pop), std::end(pop), samp2.begin(), sample_size, rng);
+  sum = std::accumulate(std::begin(out), std::end(out), 0);
+  VERIFY( sum != 0 );
+}
+
+int
+main()
+{
+  test01();
+}