🧮 Add feed decimals check to TrueUSDWithPoR in contracts-por (#1200)

Reapply changes from "Require PoR feed decimals to be equal token's decimals" commit

Author: Michał Sieczkowski

packages/contracts-por/contracts/TrueCurrencyWithPoR.sol

@@ -34,6 +34,9 @@ abstract contract TrueCurrencyWithPoR is TrueCurrency, IPoRToken {
             super._mint(account, amount);
             return;
         }
+        // Get required info about decimals.
+        // Decimals of the Proof of Reserve feed must be the same as the token's.
+        require(decimals() == AggregatorV3Interface(chainReserveFeed).decimals(), "Unexpected decimals of PoR feed");
 
         // Get latest proof-of-reserves from the feed
         (, int256 signedReserves, , uint256 updatedAt, ) = AggregatorV3Interface(chainReserveFeed).latestRoundData();
@@ -46,20 +49,10 @@ abstract contract TrueCurrencyWithPoR is TrueCurrency, IPoRToken {
         // Check the answer is fresh enough (i.e., within the specified heartbeat)
         require(block.timestamp.sub(updatedAt) <= chainReserveHeartbeat, "TrueCurrency: PoR answer too old");
 
-        // Get required info about total supply & decimals
-        uint8 trueDecimals = decimals();
-        uint8 reserveDecimals = AggregatorV3Interface(chainReserveFeed).decimals();
-        uint256 currentSupply = totalSupply();
-        // Normalise TrueCurrency & reserve decimals
-        if (trueDecimals < reserveDecimals) {
-            currentSupply = currentSupply.mul(10**uint256(reserveDecimals - trueDecimals));
-        } else if (trueDecimals > reserveDecimals) {
-            reserves = reserves.mul(10**uint256(trueDecimals - reserveDecimals));
-        }
-
+        // Get required info about total supply.
         // Check that after minting more tokens, the total supply would NOT exceed the reserves
         // reported by the latest valid proof-of-reserves feed.
-        require(currentSupply + amount <= reserves, "TrueCurrency: total supply would exceed reserves after mint");
+        require(totalSupply() + amount <= reserves, "TrueCurrency: total supply would exceed reserves after mint");
         super._mint(account, amount);
     }