Vyper: Unclear ambiguous type error

[banteg]

Version Information

• vyper Version (output of vyper --version): 0.2.4+commit.7949850
• OS: linux
• Python Version (output of python --version): Python 3.7.3
• Environment (output of pip freeze):
  asttokens==2.0.3
  pycryptodome==3.9.8
  semantic-version==2.8.5
  six==1.15.0
  vyper==0.2.4

What's your issue about?

Unclear ambiguous type error on a simple code.

# @version 0.2.4
from vyper.interfaces import ERC20

coins: constant(address[4]) = [
    0x6B175474E89094C44Da98b954EedeAC495271d0F,
    0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48,
    0xdAC17F958D2ee523a2206206994597C13D831ec7,
    0x0000000000085d4780B73119b644AE5ecd22b376,
]

@external
def recycle():
    balances: uint256[4] = empty(uint256[4])
    for i in range(4):
        balances[i] = ERC20(coins[i]).balanceOf(msg.sender)

StructureException: Ambiguous type
  contract "contracts/Recycle.vy", function "recycle", line 31:28
       30     for i in range(4):
  ---> 31         balances[i] = ERC20(coins[i]).balanceOf(msg.sender)
  ------------------------------------^

How can it be fixed?

You tell me

[iamdefinitelyahuman]

Looks like constant folding is the issue... coins[i] ends up changed to [0x6B175474E89094C44Da98b954EedeAC495271d0F, 0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48, 0xdAC17F958D2ee523a2206206994597C13D831ec7, 0x0000000000085d4780B73119b644AE5ecd22b376][i] and the type checker gets confused if it's looking at an address[4] or a Bytes[20][4].

I see two issues to address here:

1. We need to properly differentiate between address and bytes types when folding.
2. The "Ambiguous type" error should explain which types it's seeing, so that the problem is easier to diagnose.