Broken getindex with no arguments

[blegat]

The syntax x[] works if x has only one element:

julia> x = [1]
1-element Vector{Int64}:
 1

julia> x[]
1

julia> x = [1, 2]
2-element Vector{Int64}:
 1
 2

julia> x[]
ERROR: BoundsError: attempt to access 2-element Vector{Int64} at index []
Stacktrace:
 [1] throw_boundserror(A::Vector{Int64}, I::Tuple{})
   @ Base ./abstractarray.jl:703
 [2] checkbounds
   @ ./abstractarray.jl:668 [inlined]
 [3] _getindex
   @ ./abstractarray.jl:1273 [inlined]
 [4] getindex(::Vector{Int64})
   @ Base ./abstractarray.jl:1241
 [5] top-level scope
   @ REPL[4]:1

This doesn't work anywork for a vector with JuMP types because of #3237
This broke DiffOpt for instance: jump-dev/DiffOpt.jl#238

[odow]

I don't understand why x[] is meant to work iff there is one element. Is that intended behavior in Julia? Is it documented?

[blegat]

I can see it being useful in case you want to access the first element and have your code error in case there is more than one element. It's also the syntax for dereferencing a Ref so if you think of a Vector as a pointer to the first element it kind of makes sense