Inherit asset decimals in ERC4626

CHANGELOG.md

@@ -9,6 +9,8 @@
  * `GovernorCompatibilityBravo`: remove unused `using` statements. ([#3506](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3506))
  * `ERC20`: optimize `_transfer`, `_mint` and `_burn` by using `unchecked` arithmetic when possible. ([#3513](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3513))
  * `ERC20FlashMint`: add an internal `_flashFee` function for overriding. ([#3551](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3551))
+ * `ERC4626`: use the same `decimals()` as the underlying asset by default (if available). ([#3639](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3639))
+ * `ERC4626`: add internal `_initialConvertToShares` and `_initialConvertToAssets` functions to customize empty vaults behavior. ([#3639](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3639))
  * `ERC721`: optimize transfers by making approval clearing implicit instead of emitting an event. ([#3481](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3481))
  * `ERC721`: optimize burn by making approval clearing implicit instead of emitting an event. ([#3538](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3538))
  * `ERC721`: Fix balance accounting when a custom `_beforeTokenTransfer` hook results in a transfer of the token under consideration. ([#3611](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3611))
@@ -19,6 +21,10 @@
  * `ECDSA`: Remove redundant check on the `v` value. ([#3591](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3591))
  * `VestingWallet`: add `releasable` getters. ([#3580](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3580))
 
+### Breaking changes
+
+ * `ERC4626`: Conversion from shares to assets (and vice-versa) in an empty vault used to consider the possible mismatch between the underlying asset's and the vault's decimals. With the vault now using the assets decimals by default, this initial conversion rate is now set to 1-to-1. Developers overriding the vault decimals to a value that does not match the underlying asset may want to override the `_initialConvertToShares` and `_initialConvertToAssets` to replicate the previous behavior. Note that this change should not affect non-empty vault upgrades.
+
 ### Deprecations
 
  * `EIP712`: Added the file `EIP712.sol` and deprecated `draft-EIP712.sol` since the EIP is no longer a Draft. Developers are encouraged to update their imports. ([#3621](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3621))

contracts/interfaces/README.adoc

@@ -24,10 +24,12 @@ are useful to interact with third party contracts that implement them.
 - {IERC1363}
 - {IERC1820Implementer}
 - {IERC1820Registry}
+- {IERC1822Proxiable}
 - {IERC2612}
 - {IERC2981}
 - {IERC3156FlashLender}
 - {IERC3156FlashBorrower}
+- {IERC4626}
 
 == Detailed ABI
 
@@ -41,10 +43,14 @@ are useful to interact with third party contracts that implement them.
 
 {{IERC1820Registry}}
 
+{{IERC1822Proxiable}}
+
 {{IERC2612}}
 
 {{IERC2981}}
 
 {{IERC3156FlashLender}}
 
 {{IERC3156FlashBorrower}}
+
+{{IERC4626}}

contracts/mocks/ERC4626Mock.sol

@@ -4,7 +4,6 @@ pragma solidity ^0.8.0;
 
 import "../token/ERC20/extensions/ERC4626.sol";
 
-// mock class using ERC20
 contract ERC4626Mock is ERC4626 {
     constructor(
         IERC20Metadata asset,
@@ -20,3 +19,42 @@ contract ERC4626Mock is ERC4626 {
         _burn(account, amount);
     }
 }
+
+contract ERC4626DecimalMock is ERC4626Mock {
+    using Math for uint256;
+
+    uint8 private immutable _decimals;
+
+    constructor(
+        IERC20Metadata asset,
+        string memory name,
+        string memory symbol,
+        uint8 decimalsOverride
+    ) ERC4626Mock(asset, name, symbol) {
+        _decimals = decimalsOverride;
+    }
+
+    function decimals() public view virtual override returns (uint8) {
+        return _decimals;
+    }
+
+    function _initialConvertToShares(uint256 assets, Math.Rounding rounding)
+        internal
+        view
+        virtual
+        override
+        returns (uint256 shares)
+    {
+        return assets.mulDiv(10**decimals(), 10**super.decimals(), rounding);
+    }
+
+    function _initialConvertToAssets(uint256 shares, Math.Rounding rounding)
+        internal
+        view
+        virtual
+        override
+        returns (uint256 assets)
+    {
+        return shares.mulDiv(10**super.decimals(), 10**decimals(), rounding);
+    }
+}

contracts/token/ERC20/extensions/ERC4626.sol

@@ -26,15 +26,24 @@ import "../../../utils/math/Math.sol";
 abstract contract ERC4626 is ERC20, IERC4626 {
     using Math for uint256;
 
-    IERC20Metadata private immutable _asset;
+    IERC20 private immutable _asset;
 
     /**
      * @dev Set the underlying asset contract. This must be an ERC20-compatible contract (ERC20 or ERC777).
      */
-    constructor(IERC20Metadata asset_) {
+    constructor(IERC20 asset_) {
         _asset = asset_;
     }
 
+    /** @dev See {IERC4626-asset}. */
+    function decimals() public view virtual override(IERC20Metadata, ERC20) returns (uint8) {
+        try IERC20Metadata(address(_asset)).decimals() returns (uint8 value) {
+            return value;
+        } catch {
+            return super.decimals();
+        }
+    }
+
     /** @dev See {IERC4626-asset}. */
     function asset() public view virtual override returns (address) {
         return address(_asset);
@@ -153,19 +162,43 @@ abstract contract ERC4626 is ERC20, IERC4626 {
         uint256 supply = totalSupply();
         return
             (assets == 0 || supply == 0)
-                ? assets.mulDiv(10**decimals(), 10**_asset.decimals(), rounding)
+                ? _initialConvertToShares(assets, rounding)
                 : assets.mulDiv(supply, totalAssets(), rounding);
     }
 
+    /**
+     * @dev Internal conversion function (from assets to shares) to apply when the vault is empty.
+     * Note: make sure to keep this function consistent with {_initialConvertToAssets} when overriding it.
+     */
+    function _initialConvertToShares(uint256 assets, Math.Rounding /*rounding*/)
+        internal
+        view
+        virtual
+        returns (uint256 shares)
+    {
+        return assets;
+    }
+
     /**
      * @dev Internal conversion function (from shares to assets) with support for rounding direction.
      */
     function _convertToAssets(uint256 shares, Math.Rounding rounding) internal view virtual returns (uint256 assets) {
         uint256 supply = totalSupply();
         return
-            (supply == 0)
-                ? shares.mulDiv(10**_asset.decimals(), 10**decimals(), rounding)
-                : shares.mulDiv(totalAssets(), supply, rounding);
+            (supply == 0) ? _initialConvertToAssets(shares, rounding) : shares.mulDiv(totalAssets(), supply, rounding);
+    }
+
+    /**
+     * @dev Internal conversion function (from shares to assets) to apply when the vault is empty.
+     * Note: make sure to keep this function consistent with {_initialConvertToShares} when overriding it.
+     */
+    function _initialConvertToAssets(uint256 shares, Math.Rounding /*rounding*/)
+        internal
+        view
+        virtual
+        returns (uint256 assets)
+    {
+        return shares;
     }
 
     /**

contracts/token/ERC20/extensions/draft-IERC20Permit.sol

@@ -3,6 +3,8 @@
 
 pragma solidity ^0.8.0;
 
+import "../IERC20.sol";
+
 /**
  * @dev Interface of the ERC20 Permit extension allowing approvals to be made via signatures, as defined in
  * https://eips.ethereum.org/EIPS/eip-2612[EIP-2612].
@@ -11,7 +13,7 @@ pragma solidity ^0.8.0;
  * presenting a message signed by the account. By not relying on {IERC20-approve}, the token holder account doesn't
  * need to send a transaction, and thus is not required to hold Ether at all.
  */
-interface IERC20Permit {
+interface IERC20Permit is IERC20 {
     /**
      * @dev Sets `value` as the allowance of `spender` over ``owner``'s tokens,
      * given ``owner``'s signed approval.

test/token/ERC20/extensions/ERC4626.test.js

@@ -3,6 +3,7 @@ const { expect } = require('chai');
 
 const ERC20DecimalsMock = artifacts.require('ERC20DecimalsMock');
 const ERC4626Mock = artifacts.require('ERC4626Mock');
+const ERC4626DecimalMock = artifacts.require('ERC4626DecimalMock');
 
 const parseToken = (token) => (new BN(token)).mul(new BN('1000000000000'));
 const parseShare = (share) => (new BN(share)).mul(new BN('1000000000000000000'));
@@ -15,7 +16,7 @@ contract('ERC4626', function (accounts) {
 
   beforeEach(async function () {
     this.token = await ERC20DecimalsMock.new(name, symbol, 12);
-    this.vault = await ERC4626Mock.new(this.token.address, name + ' Vault', symbol + 'V');
+    this.vault = await ERC4626DecimalMock.new(this.token.address, name + ' Vault', symbol + 'V', 18);
 
     await this.token.mint(holder, web3.utils.toWei('100'));
     await this.token.approve(this.vault.address, constants.MAX_UINT256, { from: holder });
@@ -25,9 +26,18 @@ contract('ERC4626', function (accounts) {
   it('metadata', async function () {
     expect(await this.vault.name()).to.be.equal(name + ' Vault');
     expect(await this.vault.symbol()).to.be.equal(symbol + 'V');
+    expect(await this.vault.decimals()).to.be.bignumber.equal('18');
     expect(await this.vault.asset()).to.be.equal(this.token.address);
   });
 
+  it('inherit decimals if from asset', async function () {
+    for (const decimals of [ 0, 9, 12, 18, 36 ].map(web3.utils.toBN)) {
+      const token = await ERC20DecimalsMock.new('', '', decimals);
+      const vault = await ERC4626Mock.new(token.address, '', '');
+      expect(await vault.decimals()).to.be.bignumber.equal(decimals);
+    }
+  });
+
   describe('empty vault: no assets & no shares', function () {
     it('status', async function () {
       expect(await this.vault.totalAssets()).to.be.bignumber.equal('0');