Cannot decode storage item Staking.QueuedElected on block 29231

[jsdw]

The error I run into is this:

Error: Unable to decode storage staking.queuedElected:: createType(ElectionResult):: decodeU8aStruct: failed at 0x4c1650c532ed1a8641e8922aa24ade0f… on compute (index 1/4): {"_enum":{"OnChain":0,"Signed":1,"Unsigned":2}}:: Unable to create Enum via index 76, in OnChain, Signed, Unsigned

To reproduce, run these commands in a temp folder somewhere (these download a script I am using to decode historic storage and runs it with the entry and block mentioned in the title):

git clone https://github.com/jsdw/polkadot-historic-decoding-example.git
cd polkadot-historic-decoding-example/js
npm install
node index.js decode-storage-items --block 29231 --entry Staking.QueuedElected

The failure is because Staking.QueuedElected has the type ElectionResult in PJS, which looks like this:

  ElectionResult: {
    compute: 'ElectionCompute',
    slotStake: 'Balance',
    electedStashes: 'Vec<AccountId>',
    exposures: 'Vec<(AccountId, Exposure)>'
  },

I dug into old Substrate code and the correct type for this old block seems to be this:

    ElectionResult: {
      electedStashes: 'Vec<AccountId>',
      exposures: 'Vec<(AccountId, Exposure<AccountId, Balance>)>',
      compute: 'ElectionCompute'
  },

Changing the type to the above in PJS (and in the Rust decode stuff) gets everything decoding OK without any undecoded bytes etc. What I don't know is whether newer runtimes will expect a different type (eg the one currently present). However, offhand I tried some blocks, and the last block I could find where this storage entry had a value was 326344, and it still decoded correctly there (this block is halfway into runtime spec version 10).