Bug: Empty list/table pattern in switch generates invalid Lua code with { } = _exp_0

[tolpify]

When using an empty list [] (or empty table {}) as a pattern in a switch/when clause, YueScript generates invalid Lua code that attempts to destructure into an empty table literal, which causes a syntax error in Lua.
Code example:

switch x
  when [] then []
  else x

Generated lua

local _exp_0 = x
local _type_0 = type(_exp_0)
local _tab_0 = "table" == _type_0 or "userdata" == _type_0
local _match_0 = false
if _tab_0 then
	{ } = _exp_0	-- Syntax error here
	_match_0 = true
	return { }
end
if not _match_0 then
	return x
end

Environment:

$ yue --version
Yuescript version: 0.33.9

[pigpigyyy]

Do you think we should do the element number check when doing the table matching? So that empty list matching will check the table is empty.

[tolpify]

Yes, I think adding an element number check (or specifically checking if the table is empty for []/{}) is the way to go.

[pigpigyyy]

I think adding a length check to list patterns would implicitly push the semantics toward exact structural matching. If we do that for [1, 2, 3], then for consistency table patterns like {a: 1, b: 2} would also need to reject extra fields. Otherwise list matching and table matching would follow different rules.

That seems quite a bit heavier at runtime than the current partial-structure matching model.

So I’d prefer to treat [] / {} as a special-case fix for this bug, rather than changing the general matching semantics. Maybe we can make []/{} as special cases for emptiness check instead of matching any table.