Type narrowing in ternary operator not working correctly

[felixhirt]

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?

Type Checking

Expected Behaviour

In a ternary expression with type checking: "local a = type(x) == "something" and dosomething(x) or somethingelse(x)", the type should be narrowed for both cases

Actual Behaviour

The type is only narrowed for the first case (boolean here), in the second case it is still boolean or string

Reproduction steps

Copy the code

Additional Notes

No response

Log File

No response

[barrettruth]

For #2233 / #3154, I was able to replicate with:

---@param s string?
local function takes_string(s) end

---@param x string|number?
local function f(x)
    takes_string(type(x) == 'number' and tostring(x) or x)
end

I think the following MVP is appropriate:

extend the current and RHS truthiness heuristic from literals to simple direct calls ONLY whose first documented return is definitely truthy.

Scope:

• include simple name() calls like tostring(x), but only when all visible definitions document a truthy first return
• exclude helper-body inference, fallthrough analysis, chained lookups, etc.

e.g. these are in scope:

---@return string
local function as_string(v) end

---@param x string|number
local function f(x)
    local y = type(x) == 'number' and as_string(x) or x
end

to me this was broader than a tostring()-only case but still conservative enough to avoid turning tracer into a second inference engine.

@sumneko wdyt?