Forge Flatten creates duplicate declarations (unable to compile)

[daweth]

Component

Forge

Have you ensured that all of these are up to date?

• Foundry
• Foundryup

What version of Foundry are you on?

forge 0.2.0 (95a93cd 2023-08-25T00:32:44.694202000Z)

What command(s) is the bug in?

forge flatten

Operating System

macOS (Apple Silicon)

Describe the bug

To repro:

https://github.com/balancer/balancer-v2-monorepo/blob/master/pkg/vault/contracts/Vault.sol
Clone the repo and call forge flatten Vault.sol > VaultFlattened.sol

Try to compile VaultFlattened.sol in a blank forge repo

[mds1]

There's a lot of forge flatten issues that have accrued so aggregating them here, cc @Evalir @mattsse:

• forge flatten with renamed interface import does not compile #3265
• forge flatten duplication of interfaces/contract names #262 #4034 (same as this issue)
• After run forge flatten command, then I run forge build, it shows: Multiple SPDX license identifiers found in source file.  #3678 (same as Bug: Forge flatten does handle multiple differing SPDX licenses  #2344)
• forge flatten does not support qualified imports #2545
• Flatten removes "pragma experimental ABIEncoderV2;" #2378

[SidestreamColdMelon]

Hey @mds1, thanks for collecting those issues in one place!

As investigated in #4034 (comment), the flattening is actually implemented in the dependency from the same author called ethers-rs. The issue in that repo which contains description of the problem: gakonst/ethers-rs#2588.

For anyone wanting to contribute, you can start by extending "Reproducible example" provided in that issue with other use-cases and making it into a PR with a failing test.