`bytes.concat` cannot be used for initializating `constant`

[CJ42]

Description

When defining a constant variable of type bytes and initializing with some bytes that are concatenated together:

• the method abi.encodePacked(...) works ✅
• but not the method bytes.concat(...) ❌

Environment

• Compiler version: 0.8.30
• Compilation pipeline (legacy, IR, EOF): none
• Target EVM version (as per compiler settings): cancun
• Framework/IDE (e.g. Foundry, Hardhat, Remix): Foundry

Steps to Reproduce

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.30;

contract Example {

    // ✅ Compile successfully
    bytes public constant A = abi.encodePacked(
        hex"aaaa",
        hex"bbbb",
        hex"cccc"
    );

    // ❌ TypeError: Initial value for constant variable has to be compile-time constant.
    bytes public constant B = bytes.concat(
        hex"aaaa",
        hex"bbbb",
        hex"cccc"
    );

}

[olaoyesalem]

@cameel Hi I want to work on this issue

[olaoyesalem]

@CJ42 I want to work on this issue

[cameel]

Interesting. TBH I was not even aware that we allowed this for abi.encodePacked(). The fact that it was possible to make it work like this is a side-effect of our constants not being evaluated at compilation time. The value is evaluated at runtime and may or may not end up being a constant in the bytecode, depending on how well the optimizer can deal with it.

The consequence of this is that such a constants will not be usable e.g. in the calculation of a storage layout base or array sizes, even though from user's perspective there's no technical obstacle to this (the value could be calculated at compilation time). We are thinking of making compile-time evaluation stronger (see #3157 (comment)), and that would make it usable in such contexts, but only after we implement abi.encodePacked() / bytes.concat() as a compile-time function, which I wanted to treat just as the optional second step. Changing this will require us to support bytes.concat() from the very beginning. Or keep the current string behavior as a special case until we do that.

In any case, you are right, this is inconsistent and bytes.concat() should be usable in this context, at least in the state the language is in now.