getfield broadcast on empty collection doesn't infer

[goretkin]

On Version 0.7.0-DEV.3424, Commit 6afcb045b5 (and also on 0.6)

struct A
  b::Int
  c::Float64
end

As = [A(0,0) for i=1:10]

for idxs in [[1], []]
  println(ifelse(length(idxs)==0, "empty", "full"))
  @show eltype([getfield(a, :b) for a in As[idxs]]) # same as  eltype([a.b for a in As[idxs]])
  @show eltype([getfield(a, :c) for a in As[idxs]]) # same as  eltype([a.c for a in As[idxs]])
  @show eltype(getfield.(As[idxs], :b))
  @show eltype(getfield.(As[idxs], :c))
end

produces

full
eltype([getfield(a, :b) for a = As[idxs]]) = Int64
eltype([getfield(a, :c) for a = As[idxs]]) = Float64
eltype(getfield.(As[idxs], :b)) = Int64
eltype(getfield.(As[idxs], :c)) = Float64
empty
eltype([getfield(a, :b) for a = As[idxs]]) = Int64
eltype([getfield(a, :c) for a = As[idxs]]) = Float64
eltype(getfield.(As[idxs], :b)) = Union{Float64, Int64}
eltype(getfield.(As[idxs], :c)) = Union{Float64, Int64}

(the Union is unexpected)
I guess inference still works on getfield, even though it's type-unstable.

[stevengj]

getfield.(As[[]], :b) gives a union, but inference works if you call broadcast directly:

julia> @inferred broadcast(x -> getfield(x, :b), As[[]])
0-element Array{Int64,1}

The problem is not with inference per so, it is with dot calls: getfield.(B, :b) turns into broadcast(getfield, B, :b). Since the :b symbol is passed via a broadcast argument, it's quite tricky for inference to analyze the getfield call.

One possibility would be to "inline" literal symbols in dot calls, so that getfield.(B, :b) turns into broadcast(x -> getfield(x, :b), B). In principle, this may be quite easy to do by changing the definition of fl_julia_scalar in src/ast.c to return true for symbols, similar to literal numbers (which are already inlined). However, the broadcast machinery is being overhauled in #25377, and inlining of literal "scalars" will work differently (cc @mbauman).