In-kind liquidation for older markets

[jflatow]

Older markets suffer from a previous bug in the CToken contract which prevents being liquidated in the same asset that is being supplied. The markets which this issue currently affects are cBAT, cETH, cREP, cSAI, cUSDC, cWBTC, cZRX.

Given the collateral requirements, and the fact that only interest accumulation (not price risk) can put an in-kind borrower underwater, it was previously deemed non-critical to the health of the protocol to deprecate the older markets. Governors may consider other mechanisms to migrate these markets in the future, such as the deprecation of the cSAI market and its effective replacement by the newer, immutable cDAI market. Another more invasive option is that a liquidation function could be added to the Comptroller (or elsewhere, with appropriate *Allowed hook changes), which could perform the liquidate without requiring the re-entrancy.

Note: it's somewhat impossible to prevent in-kind borrowing without major complexity, as a user could supply asset A, borrow asset B, receive cTokens of asset A and then redeem the cTokens of B, effectively being the same as supply A and borrow A.

[notesbytom]

It looks like wBTC contract will be migrating to a new upgradable version soon. This should resolve the issue for wBTC?

• Legacy market migration: WBTC (from comp.xyz forum)

[TennisBowling]

@notesbytom yes, the new WBTC has this quirk fixed