Incorrect ERC721 interface

Severity: Med ID: incorrect-erc721-interface

Flags interfaces or contracts whose function signatures match an ERC721 (or ERC165) method by name and parameters but use the wrong return type.

What it does

For each function whose name and parameter types match a canonical ERC721/ERC165 method (balanceOf, ownerOf, safeTransferFrom, transferFrom, approve, setApprovalForAll, getApproved, isApprovedForAll, supportsInterface), the lint checks that the return type matches the spec. A mismatch is reported.

Why is this bad?

Non-conforming NFT contracts break marketplaces, indexers, and any protocol that relies on the ERC721 spec. A wrong return type often compiles and deploys silently but causes integration failures at runtime.

Example

Bad

solidity

interface IBadERC721 {
    function balanceOf(address) external view returns (bool);   // should be uint256
    function ownerOf(uint256) external view returns (bool);     // should be address
    function supportsInterface(bytes4) external view returns (uint256); // should be bool
}

Good

solidity

interface IERC721 {
    function balanceOf(address owner) external view returns (uint256);
    function ownerOf(uint256 tokenId) external view returns (address);
    function safeTransferFrom(address from, address to, uint256 tokenId) external;
    function transferFrom(address from, address to, uint256 tokenId) external;
    function approve(address to, uint256 tokenId) external;
    function setApprovalForAll(address operator, bool approved) external;
    function getApproved(uint256 tokenId) external view returns (address);
    function isApprovedForAll(address owner, address operator) external view returns (bool);
    function supportsInterface(bytes4 interfaceId) external view returns (bool);
}