libstdc++: Fix conversion to/from net::ip::address_v4::bytes_type

I messed up the endianness of the address_v4::bytes_type array, which
should always be in network byte order. We can just use bit_cast to
convert the _M_addr member to/from bytes_type.

libstdc++-v3/ChangeLog:

	* include/experimental/internet (address_4(const bytes_type&)):
	Use __builtin_bit_cast if available, otherwise convert to
	network byte order.
	(address_v4::to_bytes()): Likewise, but convert from network
	byte order.
	* testsuite/experimental/net/internet/address/v4/cons.cc: Fix
	incorrect tests. Check for constexpr too.
	* testsuite/experimental/net/internet/address/v4/creation.cc:
	Likewise.
	* testsuite/experimental/net/internet/address/v4/members.cc:
	Check that bytes_type is a standard-layout type.

4 files changed, 60 insertions, 20 deletions

diff --git a/libstdc++-v3/include/experimental/internet b/libstdc++-v3/include/experimental/internet
index 08bd0db..3fd2002 100644
--- a/libstdc++-v3/include/experimental/internet
+++ b/libstdc++-v3/include/experimental/internet
@@ -198,7 +198,12 @@ namespace ip
 
     constexpr
     address_v4(const bytes_type& __b)
-    : _M_addr((__b[0] << 24) | (__b[1] << 16) | (__b[2] << 8) | __b[3])
+#if __has_builtin(__builtin_bit_cast)
+    : _M_addr(__builtin_bit_cast(uint_type, __b))
+#else
+    : _M_addr(_S_hton_32((__b[0] << 24) | (__b[1] << 16)
+			   | (__b[2] << 8) | __b[3]))
+#endif
     { }
 
     explicit constexpr
@@ -227,12 +232,17 @@ namespace ip
     constexpr bytes_type
     to_bytes() const noexcept
     {
+#if __has_builtin(__builtin_bit_cast)
+      return __builtin_bit_cast(bytes_type, _M_addr);
+#else
+      auto __host = to_uint();
       return bytes_type{
-	  (_M_addr >> 24) & 0xFF,
-	  (_M_addr >> 16) & 0xFF,
-	  (_M_addr >> 8) & 0xFF,
-	  _M_addr & 0xFF
+	(__host >> 24) & 0xFF,
+	(__host >> 16) & 0xFF,
+	(__host >> 8) & 0xFF,
+	__host & 0xFF
       };
+#endif
     }
 
     constexpr uint_type
diff --git a/libstdc++-v3/testsuite/experimental/net/internet/address/v4/cons.cc b/libstdc++-v3/testsuite/experimental/net/internet/address/v4/cons.cc
index 65f2364..af9fef2 100644
--- a/libstdc++-v3/testsuite/experimental/net/internet/address/v4/cons.cc
+++ b/libstdc++-v3/testsuite/experimental/net/internet/address/v4/cons.cc
@@ -24,41 +24,45 @@
 
 using std::experimental::net::ip::address_v4;
 
-void
-test01()
+#if __cplusplus < 202002L
+// Naughty, but operator== for std::array is not constexpr until C++20.
+constexpr bool
+operator==(const address_v4::bytes_type& lhs, const address_v4::bytes_type& rhs)
 {
-  bool test __attribute__((unused)) = false;
+  return lhs[0] == rhs[0] && lhs[1] == rhs[1]
+      && lhs[2] == rhs[2] && lhs[3] == rhs[3];
+}
+#endif
 
+constexpr void
+test01()
+{
   address_v4 a0;
   VERIFY( a0.to_uint() == 0 );
   VERIFY( a0.to_bytes() == address_v4::bytes_type{} );
 }
 
-void
+constexpr void
 test02()
 {
-  bool test __attribute__((unused)) = false;
-
   address_v4 a0{ address_v4::bytes_type{} };
   VERIFY( a0.to_uint() == 0 );
   VERIFY( a0.to_bytes() == address_v4::bytes_type{} );
 
   address_v4::bytes_type b1{ 1, 2, 3, 4 };
   address_v4 a1{ b1 };
-  VERIFY( a1.to_uint() == ntohl((1 << 24) | (2 << 16) | (3 << 8) | 4) );
+  VERIFY( a1.to_uint() == ((1 << 24) | (2 << 16) | (3 << 8) | 4) );
   VERIFY( a1.to_bytes() == b1 );
 }
 
-void
+constexpr void
 test03()
 {
-  bool test __attribute__((unused)) = false;
-
   address_v4 a0{ 0u };
   VERIFY( a0.to_uint() == 0 );
   VERIFY( a0.to_bytes() == address_v4::bytes_type{} );
 
-  address_v4::uint_type u1 = ntohl((5 << 24) | (6 << 16) | (7 << 8) | 8);
+  address_v4::uint_type u1 = (5 << 24) | (6 << 16) | (7 << 8) | 8;
   address_v4 a1{ u1 };
   VERIFY( a1.to_uint() == u1 );
   VERIFY( a1.to_bytes() == address_v4::bytes_type( 5, 6, 7, 8 ) );
@@ -70,4 +74,11 @@ main()
   test01();
   test02();
   test03();
+
+  constexpr bool c = []{
+    test01();
+    test02();
+    test03();
+    return true;
+  };
 }
diff --git a/libstdc++-v3/testsuite/experimental/net/internet/address/v4/creation.cc b/libstdc++-v3/testsuite/experimental/net/internet/address/v4/creation.cc
index 441c832..84aebbb 100644
--- a/libstdc++-v3/testsuite/experimental/net/internet/address/v4/creation.cc
+++ b/libstdc++-v3/testsuite/experimental/net/internet/address/v4/creation.cc
@@ -25,7 +25,17 @@
 namespace net = std::experimental::net;
 using net::ip::address_v4;
 
-void
+#if __cplusplus < 202002L
+// Naughty, but operator== for std::array is not constexpr until C++20.
+constexpr bool
+operator==(const address_v4::bytes_type& lhs, const address_v4::bytes_type& rhs)
+{
+  return lhs[0] == rhs[0] && lhs[1] == rhs[1]
+      && lhs[2] == rhs[2] && lhs[3] == rhs[3];
+}
+#endif
+
+constexpr void
 test01()
 {
   auto a0 = make_address_v4( address_v4::bytes_type{} );
@@ -34,18 +44,18 @@ test01()
 
   address_v4::bytes_type b1{ 1, 2, 3, 4 };
   auto a1 = make_address_v4( b1 );
-  VERIFY( a1.to_uint() == ntohl((1 << 24) | (2 << 16) | (3 << 8) | 4) );
+  VERIFY( a1.to_uint() == ((1 << 24) | (2 << 16) | (3 << 8) | 4) );
   VERIFY( a1.to_bytes() == b1 );
 }
 
-void
+constexpr void
 test02()
 {
   auto a0 = net::ip::make_address_v4(0u);
   VERIFY( a0.to_uint() == 0 );
   VERIFY( a0.to_bytes() == address_v4::bytes_type{} );
 
-  address_v4::uint_type u1 = ntohl((5 << 24) | (6 << 16) | (7 << 8) | 8);
+  address_v4::uint_type u1 = ((5 << 24) | (6 << 16) | (7 << 8) | 8);
   auto a1 = net::ip::make_address_v4( u1 );
   VERIFY( a1.to_uint() == u1 );
   VERIFY( a1.to_bytes() == address_v4::bytes_type( 5, 6, 7, 8 ) );
@@ -84,4 +94,10 @@ main()
   test01();
   test02();
   test03();
+
+  constexpr bool c = []{
+    test01();
+    test02();
+    return true;
+  };
 }
diff --git a/libstdc++-v3/testsuite/experimental/net/internet/address/v4/members.cc b/libstdc++-v3/testsuite/experimental/net/internet/address/v4/members.cc
index c40a810..ac59405 100644
--- a/libstdc++-v3/testsuite/experimental/net/internet/address/v4/members.cc
+++ b/libstdc++-v3/testsuite/experimental/net/internet/address/v4/members.cc
@@ -26,6 +26,9 @@
 
 using std::experimental::net::ip::address_v4;
 
+static_assert(std::is_standard_layout<address_v4::bytes_type>::value,
+    "net::ip::address_v4::bytes_type is a standard layout type");
+
 constexpr bool
 test01()
 {