AssetHubKusama types bundle: TAssetConversion / Option<Location> fails to decode V5 cross-consensus assetId

[brenzi]

disclaimer: agent-created report. but human reproduced and confirmed the issue

Summary

Decoding a V5 cross-consensus Location (e.g. bridged-from-Polkadot USDC on
Asset Hub Kusama: parents:2, X4[GlobalConsensus(Polkadot), Parachain(1000), PalletInstance(50), GeneralIndex(1337)]) fails when it arrives via the
assetId field of a SignerPayloadJSON for AssetHubKusama (which the chain's
pallet_asset_conversion_tx_payment resolves through TAssetConversion →
Option<Location>):

createType(ExtrinsicPayload):: createType(TAssetConversion)::
decodeU8aStruct: failed at 0x090200a10f043205e514… on interior (index 2/2):
{"_enum":{"Here":"Null","X1":"[JunctionV5;1]","X2":"[JunctionV5;2]",
          "X3":"[JunctionV5;3]","X4":"[JunctionV5;4]","X5":"[JunctionV5;5]",
          "X6":"[JunctionV5;6]","X7":"[JunctionV5;7]","X8":"[JunctionV5;8]"}}::
Unable to create Enum via index 9, in Here, X1, X2, X3, X4, X5, X6, X7, X8      

The decoder reads parents=02 and Junctions::X4=04, then for the
[JunctionV5; 4] payload it reads 0x09 (which is the inner
Junction::GlobalConsensus variant) as a fresh outer Junctions variant
byte. Junctions only has variants Here, X1..X8 (indices 0–8), so index 9 is
out of range and decoding aborts.

The bytes themselves are well-formed runtime-valid SCALE; the chain accepts
them, and a tx that includes them in ChargeAssetTxPayment.value = 0x00010204090200a10f043205e514 runs fine. PAPI also encodes / decodes them
correctly. The failure is specifically in TAssetConversion's type resolution
for AssetHubKusama.

Reproduction

import { ApiPromise, WsProvider } from '@polkadot/api';
                                            
const api = await ApiPromise.create({   
  provider: new WsProvider('wss://kusama-asset-hub-rpc.polkadot.io'),
});                                                                             
                                        
// bridged-from-Polkadot USDC on KAH (V5 cross-consensus Location)              
const assetId = '0x0204090200a10f043205e514';                                   
                                        
api.createType('TAssetConversion', assetId);    // throws "Unable to create Enum
 via index 9"                                                                   
// or, equivalently, via the path the extensions exercise:
// api.createType('ExtrinsicPayload', { ..., assetId, ... })                    

Real-world impact

This is hit by every dApp using pallet_asset_conversion_tx_payment on
AssetHubKusama with a bridged asset as the fee asset. The polkadot.js extension
and SubWallet both crash their signing popups on the resulting
SignerPayloadJSON, and the React error boundary doesn't recover — every
subsequent signing prompt in the session also shows the error screen until the
browser is fully restarted.

Downstream issues with full traces, sticky-state details, and a npx serve .
HTML reproduction:

• polkadot-js/extension: Signing crashes the popup with "Unable to create Enum via index 9" when transaction's fee asset is a V5 cross-consensus Location extension#1618
• SubWallet: Signing popup crashes with "Unable to create Enum via index 9" when SignerPayload.assetId is a V5 cross-consensus Location Koniverse/SubWallet-Extension#4989

Suggested fix area

• Verify the AssetHubKusama types bundle has JunctionV5 (singular) registered
  with all V5 variants — including GlobalConsensus(StagingXcmV5NetworkId) at
  index 9 — so '[JunctionV5;4]' resolves to the correct inner type and the
  decoder reads each X4 payload entry as a Junction rather than a fresh
  Junctions.
• Confirm TAssetConversion for chains where
  pallet_asset_conversion_tx_payment::Config::AssetId = Location resolves to
  Option<StagingXcmV5Location>, not an older MultiLocation.
• Adding a regression test that round-trips a cross-consensus Location (with
  parents:2 and a GlobalConsensus junction) through TAssetConversion for
  AssetHubKusama / AssetHubPolkadot would prevent re-introduction.

Environment

• Tested chain: Asset Hub Kusama (wss://kusama-asset-hub-rpc.polkadot.io)
• Runtime spec version: 2002000