[Truffle-v5] Wrong return type for functions with multiple named return values

Adding the following function to the MetaCoin contract in examples/truffle-v5:

function foo() public pure returns (uint bar, address bam) {
  bar = 42;
  bam = address(0);
}

generates the following type bindings:

foo(txDetails?: Truffle.TransactionDetails): Promise<[BN, string]>;

Therefore, accessing these values in a test like this compiles fine:

it('should allow calling foo', async () => {
  const metaCoinInstance = await MetaCoin.new();
  let [bar, bam] = await metaCoinInstance.foo();
  assert.equal(bar.toNumber(), 42);
});

However, running the test crashes with the following Type error:

1. Contract: MetaCoin should allow calling foo: TypeError: (intermediate value) is not iterable at Context.<anonymous> (test/metacoin.ts:43:22) at processTicksAndRejections (internal/process/task_queues.js:97:5)

I believe the reason for this is that truffle v5 uses web3 v1 under the hood and thus the function above should actually compile to a Promise of a named struct rather than a Promise of a tuple. In this case the method should have the following signature:

foo(txDetails?: Truffle.TransactionDetails): Promise<{
  bar: BN;
  bam: string;
  0: BN;
  1: string;
}>;

(which is already the case when using --target=web3-v1)

As a temporary work-around, accessing the field by index compiles fine and does not crash (since the named struct also has a field with name 0 & 1):

  let result = await metaCoinInstance.foo();
  assert.equal(result[0], 42);

[JustynaBroniszewska]

I might have a similar issue. I created a repo that shows it here

TL;DR When I call a function, that is supposed to return multiple values I have a problem destructuring them:

contract Example {
    uint a = 1;
    uint b = 2;

    function getNumbers() public view returns (uint, uint) {
        return (a, b);
    }
}

This doesn’t work:

  const [a, b] = await contract.getNumbers()

…but these work:

  const { 0: a, 1: b } = await contract.getNumbers()
  const numbers = await contract.getNumbers()
  console.log(numbers[0], numbers[1])

[krzkaczor]

Released: https://github.com/ethereum-ts/TypeChain/releases/tag/typechain%404.0.1