[Bug]: math.LegacyDec nil values cannot be symmetrically marshaled and unmarshaled

[kocubinski]

Is there an existing issue for this?

• I have searched the existing issues

What happened?

#21806 documents this behavior as a test. Briefly:

math.LegacyDec{}.Marshal() -> "0"
math.LegacyZeroDec.Marshal() -> "0"
math.LegacyDec{}.Unmarshal("0") -> math.LegacyZeroDec()

Thus a nil (default) valued LegacyDec is not recoverable after marshaling. Importantly, this impacts Amino JSON marshaling and signing, which must be deterministic because the two types marshal to JSON differently:

math.LegacyDec{}.MarshalJSON() -> "0"
math.LegacyZeroDec().MarshalJSON() -> "0.000000000000000000"

Cosmos SDK Version

main

How to reproduce?

see #21806

[julienrbrt]

This makes me think of point 1 of #17750
-> #17613

I don't think we can fix it without being breaking (#17613 (comment))

[kocubinski]

Yea.. those issues are pointing to the same thing. I missed them when briefly searching for existing issues (sometimes they're hard to find). At least we should document it with a test.