Type narrowing in ternary operator not working correctly #2233

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1. sumneko

   added

   bugSomething isn't workingSomething isn't working

   on Aug 15, 2023
2. AgatZan

   marked [fix/type-checking] ternary operator incorrect type infer #3154 as a duplicate of this issue on Apr 14, 2025
3. barrettruth commented on Apr 9, 2026

   

   barrettruth

   on Apr 9, 2026

   Last edited by barrettruth

   More actions

   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?