checkbounds return value

[garrison]

I was a bit surprised to notice that the function checkbounds(A::AbstractArray{T,N}, I...) returns true if it does not throw a BoundsError.

julia> checkbounds(1:3, 4)
ERROR: BoundsError: attempt to access 1:3
  at index [4]

julia> checkbounds(1:3, 3)
true

I would have expected it to return nothing, especially since the documentation makes no mention of return value:

Throw an error if the specified indexes are not in bounds for the given array. Subtypes of AbstractArray should specialize this method if they need to provide custom bounds checking behaviors.

Note that there is another version of checkbounds that is explicitly asked to return a Bool:

julia> checkbounds(Bool, 3, 4)
false

julia> checkbounds(Bool, 3, 3)
true

Two possible solutions:

1. change checkbounds(A::AbstractArray{T,N}, I...) to return nothing if it does not throw an error. This will involve going through existing packages to make sure nobody is relying on the current behavior.
2. decide that returning true is actually no accident, and modify the documentation to mention the return value. If this decision is taken, it's especially important to make this explicit in the documentation, as subtypes of AbstractArray will want to also have their checkbounds return true to be consistent with the original interface (otherwise there can be subtle breakage).

My vote is for the first option, as returning true is redundant if no error is thrown.

[kshyatt]

Is this still a problem? What would we like to do?

[timholy]

👍 on your proposal. I'm going to strip out the "decision" tag here, and mark "up for grabs."

[Tetralux]

This appears to have been fixed. (Checked on commit 6a1483b.)

julia> checkbounds(1:3, 4)
ERROR: BoundsError: attempt to access 3-element UnitRange{Int64} at index [4]
 in throw_boundserror(::UnitRange{Int64}, ::Tuple{Int64}) at .\abstractarray.jl:326
 in checkbounds(::UnitRange{Int64}, ::Int64) at .\abstractarray.jl:283

julia> checkbounds(1:3, 3)

julia>

[mbauman]

Yup. Fixed by the revamp in #17355.