Introduce an ERC-1155 _exists() function

contracts/token/ERC1155/ERC1155.sol

@@ -31,6 +31,9 @@ contract ERC1155 is Context, ERC165, IERC1155, IERC1155MetadataURI {
     // Used as the URI for all token types by relying on ID substition, e.g. https://token-cdn-domain/{id}.json
     string private _uri;
 
+    // Mapping token ID to that token being registered as existing (1 for existing, 0 for not existing)
+    mapping (uint256 => uint256) private _tokenExists;
+
     /*
      *     bytes4(keccak256('balanceOf(address,uint256)')) == 0x00fdd58e
      *     bytes4(keccak256('balanceOfBatch(address[],uint256[])')) == 0x4e1273f4
@@ -229,6 +232,32 @@ contract ERC1155 is Context, ERC165, IERC1155, IERC1155MetadataURI {
         _uri = newuri;
     }
 
+    /**
+     * @dev Register an `id` so other contract functionality knows this token actually
+     * exists and this ID is valid. Minting will automatically call this.
+     * If `sendEvent` is true, emits an event to flag the existence, see the spec:
+     * "To broadcast the existence of a token ID with no initial balance, the contract
+     * SHOULD emit the TransferSingle event from 0x0 to 0x0, with the token creator as
+     * _operator, and a _value of 0."
+     * Minting will call this without sending an event as a similar event is sent for
+     * the minting itself already.
+     */
+    function _registerToken(uint256 id, bool sendEvent) internal virtual {
+        if (_tokenExists[id] == 0) {
+            _tokenExists[id] = 1;
+            if (sendEvent) {
+                emit TransferSingle(msg.sender, address(0), address(0), id, 0);
+            }
+        }
+    }
+
+    /**
+     * @dev Returns whether the token `id` exists. Use {_registerToken} to set this flag.
+     */
+    function _exists(uint256 id) internal view returns (bool) {
+        return _tokenExists[id] != 0;
+    }
+
     /**
      * @dev Creates `amount` tokens of token type `id`, and assigns them to `account`.
      *
@@ -247,6 +276,10 @@ contract ERC1155 is Context, ERC165, IERC1155, IERC1155MetadataURI {
 
         _beforeTokenTransfer(operator, address(0), account, _asSingletonArray(id), _asSingletonArray(amount), data);
 
+        if (!_exists(id)) {
+            _registerToken(id, false);
+        }
+
         _balances[id][account] = _balances[id][account].add(amount);
         emit TransferSingle(operator, address(0), account, id, amount);
 
@@ -271,6 +304,9 @@ contract ERC1155 is Context, ERC165, IERC1155, IERC1155MetadataURI {
         _beforeTokenTransfer(operator, address(0), to, ids, amounts, data);
 
         for (uint i = 0; i < ids.length; i++) {
+            if (!_exists(ids[i])) {
+                _registerToken(ids[i], false);
+            }
             _balances[ids[i]][to] = amounts[i].add(_balances[ids[i]][to]);
         }