(@span(byte)){...} ordered field cast permits invalid, rejects valid construction about nelua-lang

[edubart]

This is not a bug, but a feature.

For container like records (span, vector and sequence) a list of values has different meaning than a list with named fields when initializing. An example demonstrate this better:

require 'span'
require 'vector'

local a = (@span(byte)){1,2,3,4}
for i=0,<#a do
  print(a[i]) -- outputs 1,2,3,4
end

local v = (@vector(byte)){5,6,7,8}
for i=0,<#v do
  print(v[i]) -- outputs 5,6,7,8
end

But at the moment you initialize using named fields, the meaning is different, then you are really initializing its record fields. I clarified this in commit 31270b5

If you wonder why this is a feature, it's because span can be used a placeholder for functions that accept any kind of container, and even lists of values, for example:

require 'span'
require 'vector'

local function f(a: span(byte))
  for i=0,<#a do
    print(a[i]) -- outputs 5,6,7,8
  end
end

f{1,2,3,4} -- I can call here from a list of values

local v: vector(byte) = {5,6,7,8}
f(v)

local a: array(byte) = {9,10}
f(a)