In the lowering source code, we have a comment that says

;; local x, y=2, z => local x;local y;local z;y = 2

but these days, without parentheses that's actually

local (x, y)=(2, z)

i.e. a destructuring assignment to x and y.

However, even with parentheses, I'm not sure the behavior is actually what you want:

julia> global (a::Float64=1), (b::Int64=2.0), c
2.0

This returns the right-most assignment, but that seems like a bit of an arbitrary return value.

I would strongly suggest that we make that syntax case (which is unreachable without parentheses anyway) an error and only permit a single syntactic assignment per statement.

I will also note that the original code that added this behavior wanted it "for consistency with let", but let parses different these days:

julia> let a,b=2
           (a,b)
       end
ERROR: UndefVarError: `a` not defined in local scope
Suggestion: check for an assignment to a local variable that shadows a global of the same name.
Stacktrace:
 [1] top-level scope
   @ REPL[8]:2

julia> let;
           local a,b=2
           (a, b)
       end
ERROR: BoundsError: attempt to access Int64 at index [2]
Stacktrace:
 [1] indexed_iterate(I::Int64, i::Int64, state::Nothing)
   @ Base ./tuple.jl:169
 [2] top-level scope
   @ REPL[12]:2