make sure .= uses Base.identity, not a local identity

[stevengj]

As pointed out by @shashi in GiovineItalia/Gadfly.jl#864, the .= lowering was incorrectly lowering x .= y to broadcast!(identity, x, y), when in fact it should have used Base.identity. This let to a confusing error in Gadfly, which defines a local variable named identity.

@tkelman, can this be backported to 0.5.0?

[vtjnash]

lgtm

[yuyichao]

Should this be done for other builtin functions too?

julia> function f(a)
           getindex = 1
           a[1]
       end
f (generic function with 1 method)

julia> f([1])
ERROR: MethodError: objects of type Int64 are not callable
 in f(::Array{Int64,1}) at ./REPL[1]:3

[toivoh]

I think there has been an intention that you should be able to provide your own +, getindex, etc locally.

But this case is different, the identity function is just an implementation detail of .= and not the official function one should overload to add behavior to it (like + is).