LuaLS / LuaLS/lua-language-server

[Feature Request] Intersection types

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Lua
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Description

I apologize if this request has been reported before, I couldn't find any.
It would be great to be able to explicitly differentiate between union types and intersection types
e.g.

--- @class A
local a = {foo = 1}

--- @class B
local b = {bar = "hello"}

--- @param aOrB A | B
--- @param aAndB A & B
local function someFunc(aOrB, aAndB) end

someFunc(a, a) -- no warning, but 2nd argument should give a warning

--- @param a A
local function someOtherFunc(a) end

--- @type A | B -- would prefer to use A & B and not have any warnings
local aAndB = {foo = 1, bar = "hello"}
someOtherFunc(aAndB) -- Cannot assign `A|B` to parameter `A`. - `B` cannot match `A` - Type `B` cannot match `A` Lua Diagnostics. (param-type-mismatch)

While it's possible to use an explicit class with inheritance as a workaround, this is not always viable, especially when using dynamic types with generics

--- @class C: A, B
local c = {foo = 1, bar = "hello"}
someOtherFunc(aAndB) -- no warning

-- not viable when using something like this
--- @generic T1, T2
--- @param tbl1 T1
--- @param tbl2 T2
--- @return T1 | T2 -- would like to use T1 & T2 instead to avoid the warnings
local function merge(tbl1, tbl2) end

someOtherFunc(merge(a, b)) -- Cannot assign `A|B` to parameter `A`. - `B` cannot match `A` - Type `B` cannot match `A` Lua Diagnostics. (param-type-mismatch)

The suggested syntax A & B for intersection types and A | B for union types matches several other languages, such as TypeScript and PHP.

Edit: another workaround is to enable luals.github.io/wiki/settings#typeweakunioncheck. This has its own downsides, but may be helpful for people trying to avoid the param-type-mismatch warnings

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

No implementation files or tests are named. Start by tracing how Lua annotations represent unions and how the param-type-mismatch diagnostic handles them, then compare behavior with the A & B, A | B, and generic merge examples; done means intersection annotations avoid the demonstrated false warnings while unions retain their current checks.

Written by the indexing model from the issue text.

Assessment

Tech stack
lua
Domain
developer-experience, tooling
Issue type
Feature
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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