Reduce reliance on underlying decimals in ERC4626

4626 uses _asset.decimals() to calculate initial share/asset conversions

https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/extensions/ERC4626.sol#L167

EIP4626 states:

All ERC-4626 tokenized Vaults MUST implement ERC-20’s optional metadata extensions.

But EIP20 states for decimals:

OPTIONAL - This method can be used to improve usability, but interfaces and other contracts MUST NOT expect these values to be present.

So even if the vault exposes decimals for shares, the 20 spec says not to expect assets to expose decimals.

Even if both tokens expose decimals the method is for usability, not for onchain calculations.

Maybe a virtual _initialShareRatio() function could be exposed that implementing contracts can override without needing to override the entire conversion process

[Amxx]

So we could do

    function decimals() public view override(IERC20Metadata, ERC20) returns (uint8) {
        try IERC20Metadata(address(_asset)).decimals() returns (uint8 value) {
            return value;
        } catch {
            return super.decimals();
        }
    }
    
    function _convertToShares(uint256 assets, Math.Rounding rounding) internal view virtual returns (uint256 shares) {
        uint256 supply = totalSupply();
        return
            (assets == 0 || supply == 0)
                ? assets
                : assets.mulDiv(supply, totalAssets(), rounding);
    }

    function _convertToAssets(uint256 shares, Math.Rounding rounding) internal view virtual returns (uint256 assets) {
        uint256 supply = totalSupply();
        return
            (supply == 0)
                ? shares
                : shares.mulDiv(totalAssets(), supply, rounding);
    }

and no possibility to override decimal … or

    function decimals() public view virtual override(IERC20Metadata, ERC20) returns (uint8) {
        try IERC20Metadata(address(_asset)).decimals() returns (uint8 value) {
            return value;
        } catch {
            return super.decimals();
        }
    }
    
    function _convertToShares(uint256 assets, Math.Rounding rounding) internal view virtual returns (uint256 shares) {
        uint256 supply = totalSupply();
        return
            (assets == 0 || supply == 0)
                ? assets.mulDiv(10**decimals(), 10**ERC4626.decimals(), rounding)
                : assets.mulDiv(supply, totalAssets(), rounding);
    }

    function _convertToAssets(uint256 shares, Math.Rounding rounding) internal view virtual returns (uint256 assets) {
        uint256 supply = totalSupply();
        return
            (supply == 0)
                ? shares.mulDiv(10**ERC4626.decimals(), 10**decimals(), rounding)
                : shares.mulDiv(totalAssets(), supply, rounding);
    }

That way devs can still override

[pedrommaiaa]

So we could do

    function decimals() public view override(IERC20Metadata, ERC20) returns (uint8) {
        try IERC20Metadata(address(_asset)).decimals() returns (uint8 value) {
            return value;
        } catch {
            return super.decimals();
        }
    }
    
    function _convertToShares(uint256 assets, Math.Rounding rounding) internal view virtual returns (uint256 shares) {
        uint256 supply = totalSupply();
        return
            (assets == 0 || supply == 0)
                ? assets
                : assets.mulDiv(supply, totalAssets(), rounding);
    }

    function _convertToAssets(uint256 shares, Math.Rounding rounding) internal view virtual returns (uint256 assets) {
        uint256 supply = totalSupply();
        return
            (supply == 0)
                ? shares
                : shares.mulDiv(totalAssets(), supply, rounding);
    }

and no possibility to override decimal … or

    function decimals() public view virtual override(IERC20Metadata, ERC20) returns (uint8) {
        try IERC20Metadata(address(_asset)).decimals() returns (uint8 value) {
            return value;
        } catch {
            return super.decimals();
        }
    }
    
    function _convertToShares(uint256 assets, Math.Rounding rounding) internal view virtual returns (uint256 shares) {
        uint256 supply = totalSupply();
        return
            (assets == 0 || supply == 0)
                ? assets.mulDiv(10**decimals(), 10**ERC4626.decimals(), rounding)
                : assets.mulDiv(supply, totalAssets(), rounding);
    }

    function _convertToAssets(uint256 shares, Math.Rounding rounding) internal view virtual returns (uint256 assets) {
        uint256 supply = totalSupply();
        return
            (supply == 0)
                ? shares.mulDiv(10**ERC4626.decimals(), 10**decimals(), rounding)
                : shares.mulDiv(totalAssets(), supply, rounding);
    }

That way devs can still override

Really like the first option @Amxx