Looks like a code lowering bug. Can you post this to the github issue
tracker?

On Wednesday, May 28, 2014, Adam Kapor <[email protected]> wrote:

> I'm not sure what :(::) is supposed to do below, but not giving an error
> at top level suggests to me that x[1] is "an expression computing a value."
> In any case, I was surprised that += runs but doesn't `setindex!` x
>
>
> julia> x=zeros(2)
> 2-element Array{Float64,1}:
>  0.0
>  0.0
> julia> x[1]::Float64 += 1.
> 1.0
> julia> x
> 2-element Array{Float64,1}:
>  0.0
>  0.0
> julia> x[1]::Float64 = 1.
> ERROR: syntax: invalid assignment location "x[1]"
>
> julia> myf(x) = x[1]::Float64+=1
> myf (generic function with 1 method)
> julia> code_lowered(myf,(Array,))
> 1-element Array{Any,1}:
>  :($(Expr(:lambda, {:x},
> {{:#s36,:#s40},{{:x,:Any,0},{:#s36,:Any,18},{:#s40,:Float64,2}},{}},
> :(begin  # none, line 1:
>         #s40 = top(typeassert)(top(convert)(Float64,getindex(x,1)),Float64)
>         #s36 = top(typeassert)(#s40,Float64) + 1
>         #s40 = top(typeassert)(top(convert)(Float64,#s36),Float64)
>         return #s36
>     end))))
> julia> myf_noassert(x) = x[1]+=1
> myf_noassert (generic function with 1 method)
> julia> code_lowered(myf_noassert, (Array,))
> 1-element Array{Any,1}:
>  :($(Expr(:lambda, {:x}, {{:#s30},{{:x,:Any,0},{:#s30,:Any,18}},{}},
> :(begin  # none, line 1:
>         #s30 = getindex(x,1) + 1
>         setindex!(x,#s30,1)
>         return #s30
>     end))))
>
>

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