What does the unimplemented-functions warning mean?

[CodeSandwich]

According to the documentation this warning can't be generated for code that can be compiled because the compiler does the very same check. Slither always compiles code first, does it mean that it's always a false positive? What's the point of this warning then?

[elopez]

Hi! Older compiler versions (I believe <0.6) do not implement this check, so the example code compiles correctly, even though the function is unimplemented. I understand this was the original motivation behind this detector.

Additionally, on newer compilers the contract can be abstract, with no other concrete contracts inheriting from it, and therefore still compile and trigger the warning.

[CodeSandwich]

Thanks for the explanation! I didn't know that old Solidity was so unsound, that's wild. I wonder if this check should be enabled when modern Solidity is used? Depending on your approach an abstract contract that's not used anywhere is either dead code or is intended as a dependency for other codebases or future contracts. Slither is fine with unused interfaces and abstract contracts, so it clearly chooses the latter approach. It's inconistent and confusing that this behavior changes as soon as the unused abstract contract has a virtual function in it as now it should have at least 1 concrete implementation somewhere.