rework _exists() and _registerToken() slightly, do not require() it i…

…n other functions

contracts/token/ERC1155/ERC1155.sol

@@ -31,8 +31,8 @@ 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
-    mapping (uint256 => bool) private _tokenExists;
+    // 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
@@ -88,7 +88,6 @@ contract ERC1155 is Context, ERC165, IERC1155, IERC1155MetadataURI {
      */
     function balanceOf(address account, uint256 id) public view override returns (uint256) {
         require(account != address(0), "ERC1155: balance query for the zero address");
-        require(_exists(id), "ERC1155: balance query for nonexistent token");
         return _balances[id][account];
     }
 
@@ -234,28 +233,29 @@ contract ERC1155 is Context, ERC165, IERC1155, IERC1155MetadataURI {
     }
 
     /**
-     * @dev Register a token ID so other contract functionality knows this token
-     * actually exists and this ID is valid. Minting will automatically call this.
-     * Also emits an event to flag the existence, see the spec:
+     * @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."
-     * @param id uint256 ID of the token to register
+     * Minting will call this without sending an event as a similar event is sent for
+     * the minting itself already.
      */
-    function _registerToken(uint256 id) internal virtual {
-        if (!_tokenExists[id]) {
-            _tokenExists[id] = true;
-            emit TransferSingle(msg.sender, address(0), address(0), id, 0);
+    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 specified token exists. Use {_registerTokenID} to set this flag.
-     * @param id uint256 ID of the token to query the existence of
-     * @return bool whether the token exists
+     * @dev Returns whether the token `id` exists. Use {_registerToken} to set this flag.
      */
     function _exists(uint256 id) internal view returns (bool) {
-        return _tokenExists[id];
+        return _tokenExists[id] != 0;
     }
 
     /**
@@ -277,8 +277,9 @@ contract ERC1155 is Context, ERC165, IERC1155, IERC1155MetadataURI {
         _beforeTokenTransfer(operator, address(0), account, _asSingletonArray(id), _asSingletonArray(amount), data);
 
         if (!_exists(id)) {
-            _registerToken(id);
+            _registerToken(id, false);
         }
+
         _balances[id][account] = _balances[id][account].add(amount);
         emit TransferSingle(operator, address(0), account, id, amount);
 
@@ -304,7 +305,7 @@ contract ERC1155 is Context, ERC165, IERC1155, IERC1155MetadataURI {
 
         for (uint i = 0; i < ids.length; i++) {
             if (!_exists(ids[i])) {
-                _registerToken(ids[i]);
+                _registerToken(ids[i], false);
             }
             _balances[ids[i]][to] = amounts[i].add(_balances[ids[i]][to]);
         }