Remove use of std::abs in experimental::{gcd,lcm}

diff --git a/libstdc++-v3/include/experimental/numeric b/libstdc++-v3/include/experimental/numeric
index 21878f3..5089772 100644
--- a/libstdc++-v3/include/experimental/numeric
+++ b/libstdc++-v3/include/experimental/numeric
@@ -39,8 +39,8 @@ 
 # include <bits/c++14_warning.h>
 #else
 
+#include <numeric>
 #include <experimental/type_traits>
-#include <cmath>
 
 namespace std _GLIBCXX_VISIBILITY(default)
 {
@@ -52,6 +52,19 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #define __cpp_lib_experimental_gcd_lcm 201411
 
+  // std::abs is not constexpr and doesn't support unsigned integers.
+  template<typename _Tp>
+    constexpr
+    enable_if_t<__and_<is_integral<_Tp>, is_signed<_Tp>>::value, _Tp>
+    __abs(_Tp __val)
+    { return __val < 0 ? -__val : __val; }
+
+  template<typename _Tp>
+    constexpr
+    enable_if_t<__and_<is_integral<_Tp>, is_unsigned<_Tp>>::value, _Tp>
+    __abs(_Tp __val)
+    { return __val; }
+
   // Greatest common divisor
   template<typename _Mn, typename _Nn>
     constexpr common_type_t<_Mn, _Nn>
@@ -60,8 +73,8 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static_assert(is_integral<_Mn>::value, "arguments to gcd are integers");
       static_assert(is_integral<_Nn>::value, "arguments to gcd are integers");
 
-      return __m == 0 ? std::abs(__n)
-	: __n == 0 ? std::abs(__m)
+      return __m == 0 ? fundamentals_v2::__abs(__n)
+	: __n == 0 ? fundamentals_v2::__abs(__m)
 	: fundamentals_v2::gcd(__n, __m % __n);
     }
 
@@ -74,7 +87,8 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static_assert(is_integral<_Nn>::value, "arguments to lcm are integers");
 
       return (__m != 0 && __n != 0)
-	? (std::abs(__m) / fundamentals_v2::gcd(__m, __n)) * std::abs(__n)
+	? (fundamentals_v2::__abs(__m) / fundamentals_v2::gcd(__m, __n))
+	  * fundamentals_v2::__abs(__n)
 	: 0;
     }
 
diff --git a/libstdc++-v3/testsuite/experimental/numeric/77801.cc b/libstdc++-v3/testsuite/experimental/numeric/77801.cc
new file mode 100644
index 0000000..c4c8bfb
--- /dev/null
+++ b/libstdc++-v3/testsuite/experimental/numeric/77801.cc
@@ -0,0 +1,22 @@ 
+// 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-do compile { target c++14 } }
+
+#include <cstdlib>
+#include <experimental/numeric>
+constexpr int i = std::experimental::gcd(4L, 5L); // PR libstdc++/77801
diff --git a/libstdc++-v3/testsuite/experimental/numeric/gcd.cc b/libstdc++-v3/testsuite/experimental/numeric/gcd.cc
index 038f12d..b3345dc 100644
--- a/libstdc++-v3/testsuite/experimental/numeric/gcd.cc
+++ b/libstdc++-v3/testsuite/experimental/numeric/gcd.cc
@@ -25,3 +25,7 @@  static_assert(lcm(21, 6) == 42, "");
 static_assert(lcm(41, 0) == 0, "LCD with zero is zero");
 static_assert(lcm(0, 7) == 0, "LCD with zero is zero");
 static_assert(lcm(0, 0) == 0, "no division by zero");
+
+static_assert(lcm(1u, 2) == 2, "unsigned and signed");
+static_assert(lcm(3, 4u) == 12, "signed and unsigned");
+static_assert(lcm(5u, 6u) == 30, "unsigned and unsigned");
diff --git a/libstdc++-v3/testsuite/experimental/numeric/lcm.cc b/libstdc++-v3/testsuite/experimental/numeric/lcm.cc
index 2c969b0..d90c152 100644
--- a/libstdc++-v3/testsuite/experimental/numeric/lcm.cc
+++ b/libstdc++-v3/testsuite/experimental/numeric/lcm.cc
@@ -29,3 +29,6 @@  static_assert( gcd(0, 13) == 13, "GCD of any number and 0 is that number" );
 static_assert( gcd(29, 0) == 29, "GCD of any number and 0 is that number" );
 static_assert( gcd(0, 0) == 0, "" );
 
+static_assert(gcd(1u, 2) == 1, "unsigned and signed");
+static_assert(gcd(3, 4u) == 1, "signed and unsigned");
+static_assert(gcd(5u, 6u) == 1, "unsigned and unsigned");