local variables in a let block aren't optimized?

[colinfang]

x = [
    1 2 3 4 5 6;
    2 3 4 5 6 7;
]
const y = x

let
    local a = x
    @time for i in 1:10^4 reshape(a, 12) end
end
0.002322 seconds (20.00 k allocations: 781.250 KB)

let
    local a = y
    @time for i in 1:10^4 reshape(a, 12) end
end
0.000344 seconds (10.00 k allocations: 625.000 KB)

It seems the type of a isn't inferred.

[JeffBezanson]

Duplicate of #524 and #8870. This is entirely because of x, and not related to locals or let blocks.