UndefVarError if trying to shadow global variable using let

[jonas-schulze]

When working with Pluto, I like to wrap more complex cells using let and reuse simple variable names.
Within Pluto, everything works fine, but when executing the notebook as a standalone script, Julia complains about some variables not being defined.

MWE

               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.9.1 (2023-06-07)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> x = 5
5

julia> y = let
           x = 2x
           x += 1
       end
ERROR: UndefVarError: x not defined
Stacktrace:
 [1] top-level scope
   @ REPL[2]:2

Workaround

By removing the initial line break, Julia gets what I mean.
But I think this is a bug of Julia (and I don't want to use different variable names or edit the whole already-too-long notebook). 🙃

julia> y = let x = 2x
           x += 1
       end
11

julia> x
5

[MasonProtter]

No I don't think this is considered a bug, this is a core part of how name resolution works in julia. When you write

let ex1 = ex2
...
end

ex1 is meant to be resolved in the local scope of the let block, but the right hand side, ex2 is resolved in the outer scope. It's by design that if instead you write

let 
    ex1 = ex2
end

that both ex1 and ex2 are resolved in the local scope. If you have an assignment to x in the local scope, then it's assumed now that x is referring to a local variable instead of the global variable.

[jonas-schulze]

During definition of x, an rvalue of the same name obviously refers to the global identifier. By your logic, not even ex2 would exist.

Similarly, even the following continuation of my example would be illegal, which it is clearly not.

let
    y = 2x
end

[KristofferC]

No, because x is not a local variable in that example.

[MasonProtter]

I think the behaviour being requested would be a somewhat reasonable thing for us to do, and I think it’s fine to keep the issue open as a feature request.

My point was just that it’s not really a bug. as far as I’m aware this is understood and intended behaviour.