Before token transfer blocking liquidations

packages/protocol/src/abstracts/BaseVault.sol

@@ -598,22 +598,6 @@ abstract contract BaseVault is ERC20, SystemAccessControl, PausableVault, VaultP
     emit Withdraw(caller, receiver, owner, assets, shares);
   }
 
-  /**
-   * @dev Hook before all token-share transfers.
-   * Requirements:
-   * - Must check `from` can move `amount` of shares.
-   *
-   * @param from address
-   * @param to address
-   * @param amount of shares
-   */
-  function _beforeTokenTransfer(address from, address to, uint256 amount) internal view override {
-    to;
-    if (from != address(0)) {
-      require(amount <= maxRedeem(from), "Transfer more than max");
-    }
-  }
-
   /*//////////////////////////////////////////////////
       Debt management: based on IERC4626 semantics
   //////////////////////////////////////////////////*/

packages/protocol/src/vaults/borrowing/BorrowingVault.sol

@@ -59,6 +59,7 @@ contract BorrowingVault is BaseVault {
   error BorrowingVault__borrow_moreThanAllowed();
   error BorrowingVault__payback_invalidInput();
   error BorrowingVault__payback_moreThanMax();
+  error BorrowingVault__beforeTokenTransfer_moreThanMax();
   error BorrowingVault__liquidate_invalidInput();
   error BorrowingVault__liquidate_positionHealthy();
   error BorrowingVault__rebalance_invalidProvider();
@@ -150,6 +151,27 @@ contract BorrowingVault is BaseVault {
   /// Debt management overrides ///
   ///////////////////////////////*/
 
+  /**
+   * @dev Hook before all asset-share transfers.
+   * Requirements:
+   * - Must check `from` can move `amount` of shares.
+   *
+   * @param from address
+   * @param to address
+   * @param amount of shares
+   */
+  function _beforeTokenTransfer(address from, address to, uint256 amount) internal view override {
+    /**
+     * @dev Hook check activated only when called by OZ {ERC20-_transfer}
+     * User must not be able to transfer asset-shares locked as collateral
+     */
+    if (from != address(0) && to != address(0)) {
+      if (amount > maxRedeem(from)) {
+        revert BorrowingVault__beforeTokenTransfer_moreThanMax();
+      }
+    }
+  }
+
   /// @inheritdoc IVault
   function debtDecimals() public view override returns (uint8) {
     return _debtDecimals;

packages/protocol/src/vaults/yield/YieldVault.sol

@@ -159,6 +159,7 @@ contract YieldVault is BaseVault {
 
   /// @inheritdoc BaseVault
   function _computeFreeAssets(address owner) internal view override returns (uint256) {
+    // There is no restriction on asset-share movements in a {YieldVault}.
     return convertToAssets(balanceOf(owner));
   }
 

packages/protocol/test/mocking/vaults/Vault.t.sol

@@ -181,11 +181,44 @@ contract VaultUnitTests is MockingSetup, MockRoutines {
     do_depositAndBorrow(amount, borrowAmount, vault, ALICE);
 
     vm.expectRevert(BaseVault.BaseVault__withdraw_moreThanMax.selector);
-
     vm.prank(ALICE);
     vault.withdraw(amount, ALICE, ALICE);
   }
 
+  function test_tryTransferWithoutRepay(uint96 amount, uint96 borrowAmount) public {
+    uint256 minAmount = vault.minAmount();
+    vm.assume(
+      amount > minAmount && borrowAmount > minAmount && _utils_checkMaxLTV(amount, borrowAmount)
+    );
+    do_depositAndBorrow(amount, borrowAmount, vault, ALICE);
+
+    vm.expectRevert(BorrowingVault.BorrowingVault__beforeTokenTransfer_moreThanMax.selector);
+    vm.prank(ALICE);
+    vault.transfer(BOB, uint256(amount));
+  }
+
+  function test_tryTransferMaxRedeemWithoutRepay(uint96 amount, uint96 borrowAmount) public {
+    uint256 minAmount = vault.minAmount();
+    vm.assume(
+      amount > minAmount && borrowAmount > minAmount && _utils_checkMaxLTV(amount, borrowAmount)
+    );
+    do_depositAndBorrow(amount, borrowAmount, vault, ALICE);
+    uint256 maxTransferable = vault.maxRedeem(ALICE);
+
+    vm.prank(ALICE);
+    vault.transfer(BOB, maxTransferable);
+    assertEq(vault.balanceOf(BOB), maxTransferable);
+
+    uint256 nonTransferable = amount - maxTransferable;
+
+    vm.expectRevert(BorrowingVault.BorrowingVault__beforeTokenTransfer_moreThanMax.selector);
+    vm.prank(ALICE);
+    vault.transfer(BOB, nonTransferable);
+
+    // Bob's shares haven't change.
+    assertEq(vault.balanceOf(BOB), maxTransferable);
+  }
+
   function test_setMinAmount(uint256 min) public {
     vm.expectEmit(true, false, false, false);
     emit MinAmountChanged(min);