[BUG] using walrus operator needs variable declaration

[artemiogr97]

Bug description

For now it is needed to declare a variable before being usable with the walrus operator

Steps to reproduce

The following code works:

var i = 1
var j: Int
if j:=i+1:
    print(j)

It should be possible to do:

var i = 1
if j:=i+1:  # for now this causes an error here: use of unknown declaration 'j'
    print(j)

See comment here: https://github.com/modularml/mojo/pull/2742#discussion_r1615198526

System information

- wsl ubuntu 22.04 using windows 11
- mojo 2024.5.2605 (9c328b12)
- modular 0.7.2 (d0adc668)

[nmsmith]

This isn't necessarily a bug. It's unclear how the walrus operator should work in Mojo. If the variable didn't previously exist, maybe the following syntax should be required: if var j := i+1:.

[artemiogr97]

Just talking about variable declaration (not initialization), I guess guess it makes sense to have the same behavior as something like "for i in range(10):" where it is not needed to declare the "i" variable. I'm not against using var with the operator.

Personally I was just asked to create an issue for this by a maintainer.