Use an explicit registration internally to mark token IDs as existing

Author: KaiRo-at

contracts/token/ERC1155/ERC1155.sol

@@ -24,6 +24,9 @@ contract ERC1155 is ERC165, IERC1155
     // Mapping from account to operator approvals
     mapping (address => mapping(address => bool)) private _operatorApprovals;
 
+    // Mapping token ID to that token being registered as existing
+    mapping (uint256 => bool) private _tokenExists;
+
     /*
      *     bytes4(keccak256('balanceOf(address,uint256)')) == 0x00fdd58e
      *     bytes4(keccak256('balanceOfBatch(address[],uint256[])')) == 0x4e1273f4
@@ -53,6 +56,7 @@ contract ERC1155 is ERC165, IERC1155
      */
     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];
     }
 
@@ -80,6 +84,7 @@ contract ERC1155 is ERC165, IERC1155
 
         for (uint256 i = 0; i < accounts.length; ++i) {
             require(accounts[i] != address(0), "ERC1155: some address in batch balance query is zero");
+            require(_exists(id), "ERC1155: some token in batch balance query does not exist");
             batchBalances[i] = _balances[ids[i]][accounts[i]];
         }
 
@@ -194,14 +199,21 @@ contract ERC1155 is ERC165, IERC1155
     }
 
     /**
-     * @dev Returns whether the specified token exists. This is intentionally marked
-     * `virtual` as specific implementations may want to override to actually have
-     * existence be respected while the generic implementation always returns true.
+     * @dev Register a token ID so other contract functionality knows this token
+     * actually exists and this ID is valid. Minting will automatically call this.
+     * @param id uint256 ID of the token to register
+     */
+    function _registerToken(uint256 id) internal virtual {
+        return _tokenExists[id] = true;
+    }
+
+    /**
+     * @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
      */
-    function _exists(uint256 id) internal view virtual returns (bool) {
-        return true;
+    function _exists(uint256 id) internal view returns (bool) {
+        return _tokenExists[id];
     }
 
     /**
@@ -214,6 +226,9 @@ contract ERC1155 is ERC165, IERC1155
     function _mint(address to, uint256 id, uint256 value, bytes memory data) internal virtual {
         require(to != address(0), "ERC1155: mint to the zero address");
 
+        if (!_exists(id)) {
+            _registerToken(id);
+        }
         _balances[id][to] = _balances[id][to].add(value);
         emit TransferSingle(msg.sender, address(0), to, id, value);
 
@@ -232,6 +247,9 @@ contract ERC1155 is ERC165, IERC1155
         require(ids.length == values.length, "ERC1155: minted IDs and values must have same lengths");
 
         for(uint i = 0; i < ids.length; i++) {
+            if (!_exists(id)) {
+                _registerToken(id);
+            }
             _balances[ids[i]][to] = values[i].add(_balances[ids[i]][to]);
         }