Type warning when calling os.time with a table

[Caton27]

How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

Which OS are you using?

Windows

What is the issue affecting?

Type Checking

Expected Behaviour

No warning

Actual Behaviour

The following warning is appearing:

Cannot assign string|osdate to parameter osdateparam?.

• string cannot match osdateparam?
• Type string cannot match nil
• Type string cannot match osdateparam

Reproduction steps

1. Enter the following code

local tDate = os.date("*t", os.time())
-- tDate is a table, e.g. {year=2024, month=1, day=10, hour=13, min=2, sec=24, yday=10, wday=4, isdst=false}
local newTime = os.time(tDate)

2. Linter warning appears in the problems tab

Additional Notes

No response

Log File

No response

[C3pa]

Interesting, I can't reproduce this issue.

[firas-assaad]

I can reproduce it. It's because of the fix for this issue: https://github.com/LuaLS/lua-language-server/issues/2235

os.time should probably be updated to accept osdate as well.

[C3pa]

I also narrowed it down to that commit. Maybe @carsakiller can tell us why the osdate param type didn't work and needed to be duplicated as osdateparam?

[firas-assaad]

It didn't work because of the introduction of the missing field diagnostic. osdate fields returned by os.date won't have nil values, but you can pass a partial osdate to os.time. Simply updating osdate to make the fields optional would've made it difficult to work with the value returned from os.date (you'd have to check for nil), so I can understand why a separate type was needed.

[carsakiller]

@firas-assaad is correct. It was necessary to prevent excessive nil checking. This seems to be a similar problem, unfortunately. os.date can return a string or table (depending on the argument) but the server is unable to narrow the resulting type, leading to this warning that it could be a string.

I'm not sure if there is anything we can do with just the definitions to fix the string issue, as the return types of os.date are correct and the argument type of os.time is correct. We can't add a string as an option for os.time and we can't remove it as a return type of os.date either.

If we add osdate like @firas-assaad said, we can do the following:

local tDate = os.date("*t", os.time())
---@cast tDate -string
local newTime = os.time(tDate)

[firas-assaad]

I'm fine with having to cast to remove the string type. The current logic makes sense: a function may return a string, so if you pass the value to another function that only accepts tables, you'll have to add annotations to express your intent.

Now it'd be nice if we could override function definitions to say that the more specific '*t' argument returns a table whereas any other string argument returns a string, but that's probably a different story.

[carsakiller]

That is the problem talked about pretty extensively in https://github.com/LuaLS/lua-language-server/issues/1456.