Feature Request: Propagate and Validate Array Index Annotations from Immutable Variable Indexing

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?

Annotations, Type Checking, Diagnostics/Syntax Checking

Expected Behaviour

The linter should propagate and validate the array index annotations from immutable variables, recognizing that the indices are constant and understanding the types associated with these indices.

local one = 1

---@class A
---@field [1] integer
---@field [2] string

---@type A
local a = {2, 'b'}

---@type integer
local b = a[one] -- no error

Actual Behaviour

The linter assumes that the value of the indices could be anything, even when the variables are immutable and clearly annotated. This leads to incorrect type warnings and reduces the effectiveness of the type checking.

local b = a[one] -- unexpectedly produces the following error; but 'local b = a[1]' does not.

Cannot assign string|integer to integer.

• string cannot match integer
• Type string cannot match integer Lua Diagnostics. (assign-type-mismatch)

Reproduction steps

Copy source in expected behavior into VS Code with extension v3.13.1 into a file with diagnostics enabled.

Additional Notes

I use prefer arrays instead of tables in my projects to avoid the cost of hashing the key and maximize the compressibility of the Lua script with external tools. Immutable local variables, used as enums, help maintain readability and maintainability. As noted by #2721 , a table seems unnecessary.

• Using the enum annotation doesn't change the resulting behavior.

• Using the alias annotation also doesn't change the resulting behavior.

---@alias a_1 1

---@type a_1
local one = 1

• The const annotation is not supported and would be another alternative to using alias.

Log File

No response

Reproduce the provided Lua snippet in the Visual Studio Code extension with diagnostics enabled, then trace the array-index type-checking entry point responsible for the reported assign-type-mismatch warning. The change is complete when an immutable local index is validated as its constant value and the example no longer reports string|integer for the integer assignment, while direct indexing remains correct.