`isconst` has surprising and undocumented edge case behaviours about julia HOT 2 OPEN

Since a non-existent field always returns the same value (it always throws), its value is consistent

Ah, I see. The purpose of the function is to check for consistent behaviour (for the compiler, perhaps?).
From a user POV, It's not very clear from the documentation that the intended use of the function is to answer "does loading this field result in a consistent value", rather than "is this field declared as a const of this type"? The documentation seems to imply the latter. And if that's the latter, then a non-existent field is certainly not declared const because it's not declared at all.

from julia.

I think more concerning is that it also reports that abstract types have only constant-field subtypes:

julia> isconst(Any, 1)
true

Actually, this one is documented (in the code at least):

julia> isconst(BitSet, :ajkhfd) # non-existent field

Since a non-existent field always returns the same value (it always throws), its value is consistent

from julia.