Fix canMemcpy assertions that are now broken on GCC 4.

c++/src/capnp/dynamic.c++

@@ -1453,13 +1453,12 @@ DynamicValue::Reader::Reader(const Reader& other) {
     case ENUM:
     case STRUCT:
     case ANY_POINTER:
-      static_assert(kj::canMemcpy<Text::Reader>() &&
-                    kj::canMemcpy<Data::Reader>() &&
-                    kj::canMemcpy<DynamicList::Reader>() &&
-                    kj::canMemcpy<DynamicEnum>() &&
-                    kj::canMemcpy<DynamicStruct::Reader>() &&
-                    kj::canMemcpy<AnyPointer::Reader>(),
-                    "Assumptions here don't hold.");
+      KJ_ASSERT_CAN_MEMCPY(Text::Reader);
+      KJ_ASSERT_CAN_MEMCPY(Data::Reader);
+      KJ_ASSERT_CAN_MEMCPY(DynamicList::Reader);
+      KJ_ASSERT_CAN_MEMCPY(DynamicEnum);
+      KJ_ASSERT_CAN_MEMCPY(DynamicStruct::Reader);
+      KJ_ASSERT_CAN_MEMCPY(AnyPointer::Reader);
       break;
 
     case CAPABILITY:
@@ -1484,13 +1483,12 @@ DynamicValue::Reader::Reader(Reader&& other) noexcept {
     case ENUM:
     case STRUCT:
     case ANY_POINTER:
-      static_assert(kj::canMemcpy<Text::Reader>() &&
-                    kj::canMemcpy<Data::Reader>() &&
-                    kj::canMemcpy<DynamicList::Reader>() &&
-                    kj::canMemcpy<DynamicEnum>() &&
-                    kj::canMemcpy<DynamicStruct::Reader>() &&
-                    kj::canMemcpy<AnyPointer::Reader>(),
-                    "Assumptions here don't hold.");
+      KJ_ASSERT_CAN_MEMCPY(Text::Reader);
+      KJ_ASSERT_CAN_MEMCPY(Data::Reader);
+      KJ_ASSERT_CAN_MEMCPY(DynamicList::Reader);
+      KJ_ASSERT_CAN_MEMCPY(DynamicEnum);
+      KJ_ASSERT_CAN_MEMCPY(DynamicStruct::Reader);
+      KJ_ASSERT_CAN_MEMCPY(AnyPointer::Reader);
       break;
 
     case CAPABILITY:

c++/src/kj/async-io-unix.c++

@@ -735,7 +735,7 @@ Promise<Array<SocketAddress>> SocketAddress::lookupHost(
           addr.addrlen = cur->ai_addrlen;
           memcpy(&addr.addr.generic, cur->ai_addr, cur->ai_addrlen);
         }
-        static_assert(canMemcpy<SocketAddress>(), "Can't write() SocketAddress...");
+        KJ_ASSERT_CAN_MEMCPY(SocketAddress);
         output.write(&addr, sizeof(addr));
         cur = cur->ai_next;
       }

c++/src/kj/async-io-win32.c++

@@ -782,7 +782,7 @@ Promise<Array<SocketAddress>> SocketAddress::lookupHost(
           addr.addrlen = cur->ai_addrlen;
           memcpy(&addr.addr.generic, cur->ai_addr, cur->ai_addrlen);
         }
-        static_assert(canMemcpy<SocketAddress>(), "Can't write() SocketAddress...");
+        KJ_ASSERT_CAN_MEMCPY(SocketAddress);
 
         const char* data = reinterpret_cast<const char*>(&addr);
         size_t size = sizeof(addr);

c++/src/kj/common.h

@@ -468,6 +468,7 @@ constexpr bool canMemcpy() {
   // and assume we can't memcpy() at all, and must explicitly copy-construct everything.
   return false;
 }
+#define KJ_ASSERT_CAN_MEMCPY(T)
 #else
 template <typename T>
 constexpr bool canMemcpy() {
@@ -476,6 +477,8 @@ constexpr bool canMemcpy() {
 
   return __is_trivially_constructible(T, const T&) && __is_trivially_assignable(T, const T&);
 }
+#define KJ_ASSERT_CAN_MEMCPY(T) \
+  static_assert(kj::canMemcpy<T>(), "this code expects this type to be memcpy()-able");
 #endif
 
 // =======================================================================================