Julia: Broadcasting a function on an array gives an expression

FWIW I'm only seeing this with identity:

julia> sum.[1,2,3]
ERROR: MethodError: no method matching start(::Expr)
Closest candidates are:
  start(::SimpleVector) at essentials.jl:259
  start(::Base.MethodList) at reflection.jl:560
  start(::ExponentialBackOff) at error.jl:107
  ...
Stacktrace:
 [1] mapfoldl(::Base.#identity, ::Function, ::Expr) at ./reduce.jl:67
 [2] sum(::Expr) at ./reduce.jl:342
 [3] broadcast(::Function, ::Expr) at ./broadcast.jl:415

julia> identity.[1,2,3]
:([1, 2, 3])

julia> length.[1,2,3]
ERROR: MethodError: no method matching length(::Expr)
Closest candidates are:
  length(::SimpleVector) at essentials.jl:257
  length(::Base.MethodList) at reflection.jl:558
  length(::MethodTable) at reflection.jl:634
  ...
Stacktrace:
 [1] broadcast(::Function, ::Expr) at ./broadcast.jl:415

It looks like it's being lowered to broadcast(identity, :([1,2,3])).

This is not an expression that has ever meant anything. It gives various different errors in all versions of julia. We allow parsing it, so it should probably be some kind of error during lowering.