The technically correct phrasing here is it parses to a valid Julia expression :([[1] Int64[]; [2] [3]]), but evaluating that expression makes typed_hvcat throw a DimensionMismatch error. “Parseable” is a lot less than we’d hope it means.
My interpretation of concatenation is that it takes elements out of input arrays and puts them in a larger array e.g. hcat([1], [2]) == [1 2], so opting out of iteration doesn’t sound like concatenation to begin with. I’d take a macro for array literals that opts out of concatenation though, just evaluates the expressions as written for elements in a multidimensional array e.g. [1:3 2:4] makes a 3×2 Matrix{Int64} but @nocat [1:3 2:4] makes a 1×2 Matrix{UnitRange{Int64}}. Hopefully it doesn’t add the overhead of instantiating a bunch of 1 element vectors to be concatenated.