Use type info from `__newindex` annotations in metatable instead of table's

[bew]

How are you using the lua-language-server?

NeoVim

Which OS are you using?

Linux

What is the issue affecting?

Type Checking

Expected Behaviour

Accept tbl.foo = bar where tbl's keys are typed to a different type than __newindex's value type.

Actual Behaviour

Get error because bar's type is different from tbl's keys type.

Reproduction steps

With this initial code:

---@class PlugTagSpec
---@field name string
---@field desc string

---@type {[string]: PlugTagSpec}
local predefined_tags = setmetatable({}, {
  ---@param name string
  ---@param spec {name?: string, desc: string}
  __newindex = function(self, name, spec)
    spec.name = name -- add name in spec
    rawset(self, name, spec)
  end,
})

Use it with:

predefined_tags.foo = { desc = "bar" }

👉 Get error:

Missing required fields in type `PlugTagSpec`: `name`

.. even though __newindex explicitely does not require the value to be a full spec but only its description (desc).

Additional Notes

Related: #2941

Log File

No response

[tomlau10]

I think what you want is the feature request of different setter/getter type for the same field: #1298

---

If that is implemented, then we may have something like this:

---@class PlugTagSpec
---@field name string
---@field desc string

---@class PredefinedTags
---@field (read) [string] PlugTagSpec
---@field (write) [string] { name?: string, desc: string }

---@type PredefinedTags
local predefined_tags = setmetatable({}, {
    ---@param name string
    ---@param spec {name?: string, desc: string}
    __newindex = function(self, name, spec)
        spec.name = name -- add name in spec
        rawset(self, name, spec)
    end,
})

predefined_tags.foo = { desc = 'bar' }

• when setting a string field to PredefinedTags, it will use the type { name?: string, desc: string }
• while getting a string field from PredefinedTags, it will use the type PlugTagSpec

[bew]

Thanks for the answer, I saw this other issue, but since it was mentioning specific fields I'm thinking this is a separate request (focused on whole table).
If a maintainer think they are the same thing and will be fixed when the other is fixed, feel free to close 🤷

[tomlau10]

I am no maintainer so I cannot make the decision. 😕
But by just looking at your use case, I think supporting __newindex may not be generally a good idea 🤔

• __newindex will only be triggered when the field doesn't exist

local t = setmetatable({}, {
    __newindex = function (t, k, v)
        print(("triggered for %s: %s"):format(k, v))
        rawset(t, k, v)
    end,
})

t.foo = 1   --> will trigger
t.foo = 2   --> will not trigger, since there is already `t.foo`
t.foo = nil --> nothing, same as above
t.foo = 3   --> !! will trigger again

• therefore it is actually non-safe to use __newindex to emulate the different getter/setter field type 😕
• and since luals doesn't actually run the lua code, so it is very difficult for the server to know if a field is currently nil or not, let alone if it should be using the type info from __newindex

---

Anyway, another workaround (to mute the warning) that I can think of for your use case, is to use @cast

• During init stage, first use table<string, {desc: string}> to let you just assign object with a single desc field
• later cast it back to table<string, PlugTagSpec>

---@class PlugTagSpec
---@field name string
---@field desc string

---@type table<string, {desc: string}>
local predefined_tags = setmetatable({}, {
    ---@param name string
    ---@param spec {name?: string, desc: string}
    __newindex = function(self, name, spec)
      spec.name = name -- add name in spec
      rawset(self, name, spec)
    end,
})
predefined_tags.foo = { desc = "bar" } --> no more warning, as now it only requires a `desc` field

-- cast to `table<string, PlugTagSpec>`
---@cast predefined_tags table<string, PlugTagSpec>

local tag = predefined_tags.foo --> PlugTagSpec
print(tag.name, tag.desc) --> good, no undefined field warning

(I assume you define all the tags beforehand? Otherwise the variable will not be named predefined_tags 🤔 )

[bew]

therefore it is actually non-safe to use __newindex to emulate the different getter/setter field type 😕

Oh that's a good point 👀
Indeed that makes it that much harder for luals to guess when __newindex is supposed to be called...

I guess I'll do things differently then, thanks for your thoughtful answers, have a good day/night! ✨