`let` block not allowed on lhs of `.+=`

[goulart-paul]

Best explained by an example :

struct Foo{T}
    a::Vector{T}
end

n = 10
x = Foo(ones(n))
y = ones(n)

@views x.a[2:end] .+= y[2:end]  # <--- error 

The reported error is ERROR: syntax: invalid let syntax.

Confirmed on v1.7 and v1.5. Further discussion and a possible solution may be found on discourse here.

[JeffBezanson]

No, I don't think we can or should support let blocks (or other blocks) as assignment left-hand sides. @views should be able to support this though.

[simeonschaub]

It's not an actual assignment though, It's an inplace broadcast. It actually works if you use <let> .= <let> .+ foo instead.

[JeffBezanson]

Ah I guess it's a problem with broadcast lowering then?

[simeonschaub]

Yes

[vtjnash]

julia> @macroexpand @views x.a[2:end] .+= y[2:end]  # <--- error
:(let var"##S#292" = x.a
          (Base.maybeview)(var"##S#292", 2:(lastindex)(var"##S#292"))
      end .+= (Base.maybeview)(y, 2:(lastindex)(y)))