`2*_ + 1`

[jariji]

julia> @underscores (2*_ + 1)(10)
ERROR: MethodError: no method matching +(::Fix1{typeof(*), Int64, Any}, ::Int64)

The current behavior is simple and hence easy to reason about which is a big advantage. The disadvantage is that it doesn't support this slightly more complex case. Do you think this the best option or would it be better to turn this into

using Accessors
julia> @o (2*_+1)
Base.Fix2{typeof(+), Int64}(+, 1) ∘ Base.Fix1{typeof(*), Int64}(*, 2)

like Accessors.jl does?

Likewise

julia> (@underscores a[_.b])((;b=1))
ERROR: ArgumentError: invalid index: _.:b of type Fix2{typeof(getproperty), Symbol, Any}