Functions with default arguments with @boundscheck can be confusing

[KristofferC]

function load(a, idx::Int=1)
    @boundscheck checkbounds(a, idx)
    return a[idx]
end

_load(a) = @inbounds load(a)
@code_llvm _load(rand(3))

This will (perhaps confusingly) not elide the boundscheck. Arguably this is an implementation detail of default arguments leaking into the semantics.

[c42f]

I guess the workaround here is to explicitly write the lowering which the compiler is doing for you, but to add a @propagate_inbounds:

Base.@propagate_inbounds load(a) = load(a,1)

In more generality, I think this is point of confusion for various helper functions which are generated by lowering: they add a layer of indirection which can cause meta information attached by the user to not quite work as expected. (Eg keyword arguments and @inline, possibly the cause of JuliaArrays/StaticArrays.jl#659 (comment)?)

[mbauman]

Note @vchuravy's observations in #56493 — in particular that @propagate_inbounds is not itself correct because it goes one level too far, and thus prone to erroneously removing boundschecks.