string|nil passed to string parameter not flagged as error with strict settings

How are you using the lua-language-server?

Command Line

Which OS are you using?

Linux

What is the issue affecting?

Type Checking

Expected Behaviour

With the following config (see below), I expect Lua Language Server to report a type mismatch when passing a string|nil value to a function that expects a non-optional string parameter.

Specifically, this line should raise a diagnostic:

test_func(params.ip) -- where params.ip is string|nil, but test_func expects string

Actual Behaviour

No error is reported by LLS, neither in VS Code nor when using the CLI with --check.

Reproduction steps

1. Save this code in a .lua file:

---@class TestParams
---@field ip string | nil

---@param ip string
---@return string
local function test_func(ip)
    return ip
end

---@type TestParams
local params = { ip = nil }

test_func(params.ip)

2. Use the following lua-language-server config (.luarc.json or through your client):

{
  "runtime": {
    "version": "Lua 5.1"
  },
  "diagnostics": {
    "enable": true,
    "disable": [],
    "groupFileStatus": {
      "ambiguity": "Any",
      "await": "Any",
      "codestyle": "Any",
      "duplicate": "Any",
      "global": "Any",
      "luadoc": "Any",
      "redefined": "Any",
      "strict": "Any",
      "strong": "Any",
      "type-check": "Any",
      "unbalanced": "Any",
      "unused": "Any"
    },
    "groupSeverity": {
      "ambiguity": "Error",
      "await": "Error",
      "codestyle": "Error",
      "duplicate": "Error",
      "global": "Error",
      "luadoc": "Error",
      "redefined": "Error",
      "strict": "Error",
      "strong": "Error",
      "type-check": "Error",
      "unbalanced": "Error",
      "unused": "Error"
    }
  },
  "type": {
    "weakNilCheck": false
  }
}

3. Run the CLI:

lua-language-server-3.15.0-linux-x64/bin/lua-language-server --check your_file.lua

Or use VS Code extension.

Additional Notes

No response

Log File

Diagnosis completed, no problems found

Start with the supplied Lua reproducer and .luarc.json, then run the documented lua-language-server --check command to confirm the missing diagnostic. Trace the type-checking path for function arguments and add regression coverage; done means the string|nil argument is reported as incompatible with the required string parameter.