Propagate @inbounds over boundaries from lambdas, generators and broadcasts

[Uroc327]

AFAICT, the @inbounds macro is not propagated into lambdas, generators and broadcasts.

using Base: @propagate_bounds
using LinearAlgebra

@propagate_inbounds f(a, i, x) = a[i+1] - x

function foo(a)
	ret = zero(eltype(a))
	@inbounds for i in axes(a, 1)
		ret += f(a, i, 0)
	end
	ret
end

function fooG(a)
	@inbounds sum(f(a, i, 0) for i in axes(a, 1))
end

function fooλ(a)
	@inbounds sum(i -> f(a, i, 0), a)
end

function fooB(a)
	la = length(a)
	@inbounds dot([1,2,3], f.(Ref(a), la, [1,2,3]))
end

julia> a = 1:5
1:5

julia> foo(a)
20

julia> fooG(a)
ERROR: BoundsError: attempt to access 5-element UnitRange{Int64} at index [6]
Stacktrace:
 [1] getindex at ./range.jl:646 [inlined]
 [2] f at ./REPL[28]:1 [inlined]
 [3] #9 at ./none:0 [inlined]
 [4] MappingRF at ./reduce.jl:93 [inlined]
 [5] _foldl_impl at ./reduce.jl:62 [inlined]
 [6] foldl_impl at ./reduce.jl:48 [inlined]
 [7] mapfoldl_impl at ./reduce.jl:44 [inlined]
 [8] #mapfoldl#204 at ./reduce.jl:160 [inlined]
 [9] mapfoldl at ./reduce.jl:160 [inlined]
 [10] #mapreduce#208 at ./reduce.jl:287 [inlined]
 [11] mapreduce at ./reduce.jl:287 [inlined]
 [12] sum at ./reduce.jl:494 [inlined]
 [13] sum at ./reduce.jl:511 [inlined]
 [14] fooG(::UnitRange{Int64}) at ./REPL[16]:2
 [15] top-level scope at REPL[35]:1
 [16] run_repl(::REPL.AbstractREPL, ::Any) at /tmp/portage/dev-lang/julia-1.5.3/work/julia-1.5.3/usr/share/julia/stdlib/v1.5/REPL/src/REPL.jl:288

julia> fooλ(a)
ERROR: BoundsError: attempt to access 5-element UnitRange{Int64} at index [6]
Stacktrace:
 [1] getindex at ./range.jl:646 [inlined]
 [2] f at ./REPL[28]:1 [inlined]
 [3] #11 at ./REPL[17]:2 [inlined]
 [4] _mapreduce(::var"#11#12"{UnitRange{Int64}}, ::typeof(Base.add_sum), ::IndexLinear, ::UnitRange{Int64}) at ./reduce.jl:411
 [5] _mapreduce_dim at ./reducedim.jl:318 [inlined]
 [6] #mapreduce#620 at ./reducedim.jl:310 [inlined]
 [7] mapreduce at ./reducedim.jl:310 [inlined]
 [8] _sum at ./reducedim.jl:727 [inlined]
 [9] #sum#628 at ./reducedim.jl:723 [inlined]
 [10] sum at ./reducedim.jl:723 [inlined]
 [11] fooλ(::UnitRange{Int64}) at ./REPL[17]:2
 [12] top-level scope at REPL[36]:1
 [13] run_repl(::REPL.AbstractREPL, ::Any) at /tmp/portage/dev-lang/julia-1.5.3/work/julia-1.5.3/usr/share/julia/stdlib/v1.5/REPL/src/REPL.jl:288

julia> fooB(a)
ERROR: BoundsError: attempt to access 5-element UnitRange{Int64} at index [6]
Stacktrace:
 [1] getindex at ./range.jl:646 [inlined]
 [2] f at ./REPL[28]:1 [inlined]
 [3] _broadcast_getindex_evalf at ./broadcast.jl:648 [inlined]
 [4] _broadcast_getindex at ./broadcast.jl:621 [inlined]
 [5] getindex at ./broadcast.jl:575 [inlined]
 [6] macro expansion at ./broadcast.jl:932 [inlined]
 [7] macro expansion at ./simdloop.jl:77 [inlined]
 [8] copyto! at ./broadcast.jl:931 [inlined]
 [9] copyto! at ./broadcast.jl:886 [inlined]
 [10] copy at ./broadcast.jl:862 [inlined]
 [11] materialize at ./broadcast.jl:837 [inlined]
 [12] fooB(::UnitRange{Int64}) at ./REPL[18]:3
 [13] top-level scope at REPL[37]:1
 [14] run_repl(::REPL.AbstractREPL, ::Any) at /tmp/portage/dev-lang/julia-1.5.3/work/julia-1.5.3/usr/share/julia/stdlib/v1.5/REPL/src/REPL.jl:288

Although this can be circumvented for Generators and Lambdas by moving the @inbounds macro into the Generator/Lambda, this is rather unpractical in functions which are @propagate_inbounds themselves. The developer does not know, if he needs to add a @inbounds macro statically.

The only workaround I know for the broadcasting part is to add an explicit @inbounds inside the broadcasted function. This is hard if the function shouldn't/can't change and the same problem in propagated contexts applies.

How can I propagate disabled bounds checks into generators, lambdas and broadcasts. And how can I do this, when the foo functions are not explicitely marked @inbounds, but are @propagate_inbounds marked themselves?

[mbauman]

Discussion on discourse here, somewhat related to #18773. I'm afraid this isn't something we want in general.