string.unpack vararg returns

[Bilal2453]

How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

Which OS are you using?

Linux

What is the issue affecting?

Annotations, Type Checking, Completion

Expected Behaviour

The current string.unpack annotations (for the returns) are as following:

---@param fmt  string
---@param s    string
---@param pos? integer
---@return any ...
---@return integer offset
---@nodiscard
function string.unpack(fmt, s, pos) end

The function will indeed return any number of any returns followed by a single integer return for the new offset. As such you would expect that in the following example code, all of the returns are any except the last return be integer:

local a, b, c, d, offset = string.unpack('I2I2I2I2', "some binary data")

Actual Behaviour

Instead of the a, b, c, d being typed as any and offset being typed as integer, we instead see that a is typed as any, b is typed as integer, and c, d, offset are typed as nil.

This happen because it is not supported for a function to return a vararg followed by any return; it is always that vararg returns are the last return. Because of that, only the first return is typed as any and from there the second @return takes.

Reproduction steps

1. Copy the example code.
2. Hover over each return variable to notice the type.

Additional Notes

No response

Log File

No response

[tomlau10]

Just come across this issue in one of the older projects in my team, which heavily uses string.unpack when decoding client data.

I guess there is no possible or logical way to specify this ---@return integer offset, given that the position of this offset depends on the actual unpack pattern. Also not all the time we will extract this offset when doing string.unpack, so even if assigning the last variable in the assignment statement to be an integer, it will not work either. 😕

---

I suggest removing the ---@return integer offset all together, just let all return values be any. And users explicitly cast the offset variable to integer if they want.

---@param fmt  string
---@param s    string
---@param pos? integer
---@return any ...
---@nodiscard
function string.unpack(fmt, s, pos) end

[Bilal2453]

Making it any is a good first step. It is actually possible to extract the types from the pattern, similar to how we extract the types from FFI definitions, etc.

It can also be kept as it is, and the user casts it later, it should still be possible to override this with a cast.