Update MerkleProof note clarifying empty set definition (#5144)

Co-authored-by: cairo <cairoeth@protonmail.com>
Co-authored-by: sudo rm -rf --no-preserve-root / <pcaversaccio@users.noreply.github.com>

contracts/utils/cryptography/MerkleProof.sol

@@ -169,9 +169,8 @@ library MerkleProof {
      *
      * CAUTION: Not all Merkle trees admit multiproofs. See {processMultiProof} for details.
      *
-     * NOTE: The _empty set_ (i.e. the case where `proof.length == 0 && leaves.length == 0`) is considered a noop,
+     * NOTE: Consider the case where `root == proof[0] && leaves.length == 0` as it will return `true`.
-     * and therefore a valid multiproof (i.e. it returns `true`). Consider disallowing this case if you're not
+     * The `leaves` must be validated independently. See {processMultiProof}.
-     * validating the leaves elsewhere.
      */
     function multiProofVerify(
         bytes32[] memory proof,
@@ -193,6 +192,10 @@ library MerkleProof {
      * CAUTION: Not all Merkle trees admit multiproofs. To use multiproofs, it is sufficient to ensure that: 1) the tree
      * is complete (but not necessarily perfect), 2) the leaves to be proven are in the opposite order they are in the
      * tree (i.e., as seen from right to left starting at the deepest layer and continuing at the next layer).
+     *
+     * NOTE: The _empty set_ (i.e. the case where `proof.length == 1 && leaves.length == 0`) is considered a no-op,
+     * and therefore a valid multiproof (i.e. it returns `proof[0]`). Consider disallowing this case if you're not
+     * validating the leaves elsewhere.
      */
     function processMultiProof(
         bytes32[] memory proof,
@@ -252,9 +255,8 @@ library MerkleProof {
      *
      * CAUTION: Not all Merkle trees admit multiproofs. See {processMultiProof} for details.
      *
-     * NOTE: The _empty set_ (i.e. the case where `proof.length == 0 && leaves.length == 0`) is considered a noop,
+     * NOTE: Consider the case where `root == proof[0] && leaves.length == 0` as it will return `true`.
-     * and therefore a valid multiproof (i.e. it returns `true`). Consider disallowing this case if you're not
+     * The `leaves` must be validated independently. See {processMultiProof}.
-     * validating the leaves elsewhere.
      */
     function multiProofVerify(
         bytes32[] memory proof,
@@ -277,6 +279,10 @@ library MerkleProof {
      * CAUTION: Not all Merkle trees admit multiproofs. To use multiproofs, it is sufficient to ensure that: 1) the tree
      * is complete (but not necessarily perfect), 2) the leaves to be proven are in the opposite order they are in the
      * tree (i.e., as seen from right to left starting at the deepest layer and continuing at the next layer).
+     *
+     * NOTE: The _empty set_ (i.e. the case where `proof.length == 1 && leaves.length == 0`) is considered a no-op,
+     * and therefore a valid multiproof (i.e. it returns `proof[0]`). Consider disallowing this case if you're not
+     * validating the leaves elsewhere.
      */
     function processMultiProof(
         bytes32[] memory proof,
@@ -337,9 +343,8 @@ library MerkleProof {
      *
      * CAUTION: Not all Merkle trees admit multiproofs. See {processMultiProof} for details.
      *
-     * NOTE: The _empty set_ (i.e. the case where `proof.length == 0 && leaves.length == 0`) is considered a noop,
+     * NOTE: Consider the case where `root == proof[0] && leaves.length == 0` as it will return `true`.
-     * and therefore a valid multiproof (i.e. it returns `true`). Consider disallowing this case if you're not
+     * The `leaves` must be validated independently. See {processMultiProofCalldata}.
-     * validating the leaves elsewhere.
      */
     function multiProofVerifyCalldata(
         bytes32[] calldata proof,
@@ -361,6 +366,10 @@ library MerkleProof {
      * CAUTION: Not all Merkle trees admit multiproofs. To use multiproofs, it is sufficient to ensure that: 1) the tree
      * is complete (but not necessarily perfect), 2) the leaves to be proven are in the opposite order they are in the
      * tree (i.e., as seen from right to left starting at the deepest layer and continuing at the next layer).
+     *
+     * NOTE: The _empty set_ (i.e. the case where `proof.length == 1 && leaves.length == 0`) is considered a no-op,
+     * and therefore a valid multiproof (i.e. it returns `proof[0]`). Consider disallowing this case if you're not
+     * validating the leaves elsewhere.
      */
     function processMultiProofCalldata(
         bytes32[] calldata proof,
@@ -420,9 +429,8 @@ library MerkleProof {
      *
      * CAUTION: Not all Merkle trees admit multiproofs. See {processMultiProof} for details.
      *
-     * NOTE: The _empty set_ (i.e. the case where `proof.length == 0 && leaves.length == 0`) is considered a noop,
+     * NOTE: Consider the case where `root == proof[0] && leaves.length == 0` as it will return `true`.
-     * and therefore a valid multiproof (i.e. it returns `true`). Consider disallowing this case if you're not
+     * The `leaves` must be validated independently. See {processMultiProofCalldata}.
-     * validating the leaves elsewhere.
      */
     function multiProofVerifyCalldata(
         bytes32[] calldata proof,
@@ -445,6 +453,10 @@ library MerkleProof {
      * CAUTION: Not all Merkle trees admit multiproofs. To use multiproofs, it is sufficient to ensure that: 1) the tree
      * is complete (but not necessarily perfect), 2) the leaves to be proven are in the opposite order they are in the
      * tree (i.e., as seen from right to left starting at the deepest layer and continuing at the next layer).
+     *
+     * NOTE: The _empty set_ (i.e. the case where `proof.length == 1 && leaves.length == 0`) is considered a no-op,
+     * and therefore a valid multiproof (i.e. it returns `proof[0]`). Consider disallowing this case if you're not
+     * validating the leaves elsewhere.
      */
     function processMultiProofCalldata(
         bytes32[] calldata proof,

scripts/generate/templates/MerkleProof.js

@@ -89,9 +89,8 @@ const templateMultiProof = ({ suffix, location, visibility, hash }) => `\
  *
  * CAUTION: Not all Merkle trees admit multiproofs. See {processMultiProof} for details.
  *
- * NOTE: The _empty set_ (i.e. the case where \`proof.length == 0 && leaves.length == 0\`) is considered a noop,
+ * NOTE: Consider the case where \`root == proof[0] && leaves.length == 0\` as it will return \`true\`.
- * and therefore a valid multiproof (i.e. it returns \`true\`). Consider disallowing this case if you're not
+ * The \`leaves\` must be validated independently. See {processMultiProof${suffix}}.
- * validating the leaves elsewhere.
  */
 function multiProofVerify${suffix}(${formatArgsMultiline(
   `bytes32[] ${location} proof`,
@@ -114,6 +113,10 @@ function multiProofVerify${suffix}(${formatArgsMultiline(
  * CAUTION: Not all Merkle trees admit multiproofs. To use multiproofs, it is sufficient to ensure that: 1) the tree
  * is complete (but not necessarily perfect), 2) the leaves to be proven are in the opposite order they are in the
  * tree (i.e., as seen from right to left starting at the deepest layer and continuing at the next layer).
+ *
+ * NOTE: The _empty set_ (i.e. the case where \`proof.length == 1 && leaves.length == 0\`) is considered a no-op,
+ * and therefore a valid multiproof (i.e. it returns \`proof[0]\`). Consider disallowing this case if you're not
+ * validating the leaves elsewhere.
  */
 function processMultiProof${suffix}(${formatArgsMultiline(
   `bytes32[] ${location} proof`,