Inconsistent update assignment behavior with 'with'

[ThaisRobba]

If I do

my_str = "say "
with my_str = my_str .. "something"
    \sub(1, 10)

It works. But if I do

my_str = "say "
with my_str ..= "something"
    \sub(1, 10)

It does not, as it breaks the compilation. Behavior across assignments should be consistent as to avoid surprises (or having to look at the outputted Lua code).