Optional not removed in if/elseif block for field in filter

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?

Type Checking

Expected Behaviour

---@type { n: number? }
local t = { n = 1 }

if t.n then
	_ = t.n -- (field) t.n: number
else
	_ = t.n -- (field) t.n: number?
end

Actual Behaviour

---@type number?
local n = 0

if n then
	_ = n -- local t: number
else
	_ = n -- local t: nil
end

-- but

---@type { n: number? }
local t = { n = 1 }

if t.n then
	_ = t.n -- (field) t.n: number? !!!
else
	_ = t.n -- (field) t.n: number?
end

Additional Notes

I couldn't immediately understand where exactly this type casting is happening

Looks like here
https://github.com/LuaLS/lua-language-server/blob/34ff9d3ca730bc28879ab2b0c1a49f2c5480f9a3/script/vm/tracer.lua#L240-L243
but adding debug log nothing came out

I would be grateful for the information, I could try to fix it myself

Reproduce the two narrowing examples in the Visual Studio Code extension, then inspect script/vm/tracer.lua around lines 240-243, the location mentioned in the issue. Done means the field access t.n is narrowed to number in the if branch while remaining number? in the else branch, matching the local-variable behavior.