Check that "getWithdrawableAmount" works with non-18-decimal tokens

[PaulRBerg]

The issue with the current implementation is that it expects the token amounts to be compatible with the PRBMath UD60x18 number format (which uses 18 decimals):

https://github.com/sablierhq/v2-core/blob/1527035a7dfdafecab971ad7d41889c5868022c6/src/SablierV2Linear.sol#L89

This won't work with non-18-decimal tokens, such as USDC, which uses 6 decimals.

[andreivladbrg]

I wrote a test for this. It works fine, see: bf4e8d8 . You can clone & test.

[PaulRBerg]

@andreivladbrg there may be values for which the current approach still works, but there are certainly many other values for which it breaks.

The solution is to normalize the token amounts to have 18 decimals too. My worry though is that the implementation isn't as easy at it seems - we might have to cache the token decimals so we don't make a query to the ERC-20 token contract on every call to getWithdrawableAmount.

[PaulRBerg]

I think I was wrong when I said that there may be "other values for which it breaks".

The reason the math still works out is that the scaling factors cancel out. Let's take USDC as an example. If we multiply depositAmount by 10^12, and then we divide the resulting product by 10^12, we reach the same result as if we had not multiplied and divided by 10^12 in the first place.

That said, there might still be some precision gains if we normalize the amounts to 18 decimals. We have to investigate this - I'll mark this issue as stale until we have clarity.

[PaulRBerg]

As per the recent work I did in #78, it looks like getWithdrawableAmount works just fine when the token has 6 decimals.

I will keep this issue open until we knock out #22. Fuzzing the number of decimals of the ERC-20 tokens used for streaming would reveal whether the current approach scales.

[PaulRBerg]

I have closed this since in #220 I have added fuzz tests for the grossDepositAmount param.

I realized that we don't actually need to test tokens with different decimals. We are never actually pulling the token decimals in our code base - what we need to test is simply different deposit amounts, which we are doing now.