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Lua Language Server Fails to Infer Type When Passing Multiple Return Values to Generic Function #3007
Description
How are you using the lua-language-server?
NeoVim
Which OS are you using?
MacOS
What is the issue affecting?
Type Checking
Expected Behaviour
---@return integer
---@return string
local f = function()
return 1, "a"
end
---@generic T
---@generic U
---@param t T
---@param u U
---@return U
local g = function(t, u)
return u
end
local x = g(f())
When the function g is called with the multiple return values of f, the Lua Language Server should infer the type of x based on the second return value of f (string), corresponding to the second generic type parameter (U) in g.
Actual Behaviour
The Lua Language Server fails to infer the type of x. Instead, the type is left unknown.
Reproduction steps
- Write the following Lua code in an editor with the Lua Language Server enabled:
---@return integer
---@return string
local f = function()
return 1, "a"
end
---@generic T
---@generic U
---@param t T
---@param u U
---@return U
local g = function(t, u)
return u
end
local x = g(f())- Observe the type inference for x.
- Note that the type of x is not inferred as string as expected.
Additional Notes
No response
Log File
No response
Activity
litoj commentedon Nov 6, 2025
I have a similar problem. In my case, when I wrap the return values in a table that I'm passing to a function it says that I am passing it only a table filled on the first index.
However, when I save it to a variable first, it still thinks there is only the first index, but for some reason passes the variable to the function with no error.
This ends up being very inconvenient, because even with the diagnostic ignored it still believes there is no match for the parameters therefore the return type of function that received the table is unknown. And that breaks the type inference and completion for everything using that and so on.