libstdc++: Implement some missing functions for net::ip::network_v6

libstdc++-v3/ChangeLog:

	* include/experimental/internet (network_v6::network): Define.
	(network_v6::hosts): Finish implementing.
	(network_v6::to_string): Do not concatenate std::string to
	arbitrary std::basic_string specialization.
	* testsuite/experimental/net/internet/network/v6/cons.cc: New
	test.

libstdc++-v3/include/experimental/internet

@@ -1347,17 +1347,35 @@ namespace ip
     constexpr address_v6 address() const noexcept { return _M_addr; }
     constexpr int prefix_length() const noexcept { return _M_prefix_len; }
 
-    constexpr address_v6 network() const noexcept; // TODO
+    _GLIBCXX17_CONSTEXPR address_v6
+    network() const noexcept
+    {
+      address_v6::bytes_type __bytes = _M_addr.to_bytes();
+      int __nbytes = (_M_prefix_len + 7) / 8;
+      for (int __n = __nbytes; __n < 16; ++__n)
+	__bytes[__n] = 0;
+      if (int __zbits = (__nbytes * 8) - _M_prefix_len)
+	__bytes[__nbytes - 1] &= 0xFF << __zbits;
+      return address_v6(__bytes, _M_addr.scope_id());
+    }
 
     address_v6_range
     hosts() const noexcept
     {
       if (is_host())
 	return { address(), *++address_v6_iterator(address()) };
-      return {}; // { network(), XXX broadcast() XXX }; // TODO
+
+      address_v6::bytes_type __bytes = _M_addr.to_bytes();
+      int __nbytes = (_M_prefix_len + 7) / 8;
+      for (int __n = __nbytes; __n < 16; ++__n)
+	__bytes[__n] = 0xFF;
+      if (int __bits = (__nbytes * 8) - _M_prefix_len)
+	__bytes[__nbytes - 1] |= (1 << __bits) - 1;
+      address_v6 __last(__bytes, _M_addr.scope_id());
+      return { network(), *++address_v6_iterator(__last) };
     }
 
-    constexpr network_v6
+    _GLIBCXX17_CONSTEXPR network_v6
     canonical() const noexcept
     { return network_v6{network(), prefix_length()}; }
 
@@ -1379,7 +1397,7 @@ namespace ip
       to_string(const _Allocator& __a = _Allocator()) const
       {
 	return address().to_string(__a) + '/'
-	  + std::to_string(prefix_length());
+		 + std::to_string(prefix_length()).c_str();
       }
 
   private:

libstdc++-v3/testsuite/experimental/net/internet/network/v6/cons.cc

@@ -0,0 +1,74 @@
+// { dg-do run { target c++14 } }
+// { dg-require-effective-target net_ts_ip }
+// { dg-add-options net_ts }
+
+#include <experimental/internet>
+#include <stdexcept>
+#include <testsuite_hooks.h>
+
+using std::experimental::net::ip::network_v6;
+using std::experimental::net::ip::address_v6;
+
+constexpr void
+test01()
+{
+  network_v6 n0;
+  VERIFY( n0.address().is_unspecified() );
+  VERIFY( n0.prefix_length() == 0 );
+}
+
+constexpr void
+test02()
+{
+  address_v6 a0;
+  network_v6 n0{ a0, 0 };
+  VERIFY( n0.address() == a0 );
+  VERIFY( n0.prefix_length() == 0 );
+
+  address_v6 a1{ address_v6::bytes_type{ 1, 2, 4, 8, 16, 32, 64, 128,
+					 3, 7, 47, 71, 83, 165, 199, 255 } };
+  network_v6 n1{ a1, 99};
+  VERIFY( n1.address() == a1 );
+  VERIFY( n1.prefix_length() == 99 );
+}
+
+void
+test02_errors()
+{
+  address_v6 a0;
+  try
+  {
+    network_v6{a0, -1};
+    VERIFY(false);
+  }
+  catch(const std::out_of_range&)
+  {
+  }
+
+  try
+  {
+    network_v6{a0, 129};
+    VERIFY(false);
+  }
+  catch(const std::out_of_range&)
+  {
+  }
+}
+
+constexpr bool
+test_constexpr()
+{
+  test01();
+  test02();
+  return true;
+}
+
+int
+main()
+{
+  test01();
+  test02();
+  test02_errors();
+
+  static_assert( test_constexpr(), "valid in constant expressions" );
+}