Infer overload based on argument values

[sapphyrus]

Currently, multiple "overloaded" annotations for the same function seem to always be combined into one union, instead of selecting the overload based on the actual, known argument values. For example:

---@meta

---@class event_a
---@field prop_a integer
local event_a = {}

---@class event_b
---@field prop_b integer
local event_b = {}

---@param event_name "a"
---@param callback fun(evt: event_a)
function register_callback(event_name, callback) end

---@param event_name "b"
---@param callback fun(evt: event_b)
function register_callback(event_name, callback) end

The different values for the event_name parameter could be used to deduce the callback type, and therefore the type of the evt parameter. For example when used like this, the type of evt should be known to be event_b:

register_callback("a", function(evt)
    print(evt.prop_a) -- valid
    print(evt.prop_b) -- not valid (Undefined field)
end)

[carsakiller]

There are a bunch of issues referencing this. I think the title of this issue best suits the issue. However, the use case in this issue is almost the exact same as in #1456, which discusses changing the way things are overloaded.

Similar:
#1343
#1410

[carsakiller]

This does indeed seem to be working now.