Nested memory arrays not implemented in ABICoder V1

[Subway2023]

Description

When I use abicoderV1 to encode a two-dimensional array, it executes normally. However, when decoding, a code generation error occurs.

Environment

• Compiler version: 0.8.26
• Target EVM version (as per compiler settings): None
• Framework/IDE (e.g. Truffle or Remix): None
• EVM execution environment / backend / blockchain client: None
• Operating system: Linux

Steps to Reproduce

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
pragma abicoder v1;
contract BuggyContract {
    int[5][] public a;
    function initialize() public {
        a.push([int(1), int(-45), int(-78), int(56), int(60)]);
        a.push([int(-1), int(42), int(334), int(-45455), int(-446)]);
        a.push([int(1), int(10), int(-5435), int(45), int(-7)]);

        bytes memory encodedA2 = abi.encode(a);
        int[5][] memory decodedA2 = abi.decode(encodedA2, (int[5][]));

    }
}

Compile

solc test.sol

Compiler run successful. No output generated.

Codegen

solc test.sol --bin

Unimplemented feature:
/solidity/libsolidity/codegen/CompilerUtils.cpp(287): Throw in function void solidity::frontend::CompilerUtils::abiDecode(const TypePointers&, bool)
Dynamic exception type: boost::wrapexcept<solidity::langutil::UnimplementedFeatureError>
std::exception::what: Nested memory arrays not yet implemented here.
[solidity::util::tag_comment*] = Nested memory arrays not yet implemented here.

Analyse

I believe this is a bug for the following two reasons:

1. During the compilation phase, there is no error indicating that abicoderV1 does not support two-dimensional arrays. If it were a struct, solc would directly throw an error to indicate this.
2. abicoderV1 supports encoding two-dimensional arrays, but it does not support decoding them back into two-dimensional arrays.