table newindex shows invalid need check for nil warning if key is false

How are you using the lua-language-server?

NeoVim

Which OS are you using?

Windows

What is the issue affecting?

Type Checking

Expected Behaviour

t[key] = {} should throw no warning if key is not nil.

Actual Behaviour

t[key] = {} throws an invalid warning if key is false but not nil.

Reproduction steps

local CACHE = {}
CACHE[false] = {} -- ok
local key = false
CACHE[key] = {} -- should be ok as well, but throws invalid warning

Additional Notes

This happened to me in a more complex method where I ensured a key is not nil, by redefining the parameter:

function apicall(input)
    local key = input or false
    -- more stuff...
    cache[key] = {}
    return cache[key]
end

I'd like to note an annoyance when it comes to casting, the following casts all throw warnings, even if explicitly casting the variable, requiring me to invent a new variable (or forcing me disable redefining local):

---@param input string?
function apicall(input)
    -- this cast does nothing and also throws no warning
    ---@cast input +false
    -- this cast warns that it can't cast string? to string|false
    ---@cast input string|false

    -- this is what I do, it just throws an info warning, which I've disabled:
    ---@type string | false
    local input = input or false
end

Log File

No response

analysis

The logic of need-check-nil diagnostics currently checks for node:hasFalsy(), which includes optional / nil / false and this seems to be the root problem 🤔 It should only check for optional and nil I believe?

lua-language-server/script/core/diagnostics/need-check-nil.lua

Line 40 in 0187ddf

if node:hasFalsy() and not vm.getInfer(src):hasType(uri, 'any') then

lua-language-server/script/vm/node.lua

Lines 106 to 116 in 0187ddf

    
           function mt:hasFalsy() 
        
               if self.optional then 
        
                   return true 
        
               end 
        
               for _, c in ipairs(self) do 
        
                   if c.type == 'nil' 
        
                   or (c.type == 'global' and c.cate == 'type' and c.name == 'nil') 
        
                   or (c.type == 'global' and c.cate == 'type' and c.name == 'false') 
        
                   or (c.type == 'boolean' and c[1] == false) 
        
                   or (c.type == 'doc.type.boolean' and c[1] == false) then 
        
                       return true

possible fix

To address this issue, I tested that we can add a node:hasNil() method:

---@return boolean
function mt:hasNil()
    if self.optional then
        return true
    end
    for _, c in ipairs(self) do
        if c.type == 'nil'
        or (c.type == 'global' and c.cate == 'type' and c.name == 'nil')
        then
            return true
        end
    end
    return false
end

and then in diagnostics/need-check-nil.lua change that line to

if node:hasNil() and not vm.getInfer(src):hasType(uri, 'any') then

This can fix the case in Reproduction steps

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table newindex shows invalid `need check for nil` warning if key is `false` #3359

How are you using the lua-language-server?

Which OS are you using?

What is the issue affecting?

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Actual Behaviour

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	function mt:hasFalsy()
	if self.optional then
	return true
	end
	for _, c in ipairs(self) do
	if c.type == 'nil'
	or (c.type == 'global' and c.cate == 'type' and c.name == 'nil')
	or (c.type == 'global' and c.cate == 'type' and c.name == 'false')
	or (c.type == 'boolean' and c[1] == false)
	or (c.type == 'doc.type.boolean' and c[1] == false) then
	return true

table newindex shows invalid need check for nil warning if key is false #3359

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How are you using the lua-language-server?

Which OS are you using?

What is the issue affecting?

Expected Behaviour

Actual Behaviour

Reproduction steps

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tomlau10 commented on Feb 24, 2026

analysis

possible fix

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table newindex shows invalid `need check for nil` warning if key is `false` #3359