Type inference for functions with methods

[toots]

See: #4284

^{Originally posted by martinkirch December 24, 2024}
Using v2.3, I'm trying to implement a function able to check and update the given reference. A simplified version looks like:

next_track = ref({title="", artist="", rid=-1, initial_uri= ""})
next_jingle = ref({title="", artist="", rid=-1, initial_uri= ""})

def update_next_track(r)
    if r().rid != 0 then
    r := {title="bla", artist="blo", rid=0, initial_uri="http://bla"}
    end
end

update_next_track(next_track)
update_next_track(next_jingle)

but this results in a type error

line 6, char 4:
    r := {title="bla", artist="blo", rid=0, initial_uri="http://bla"}

Error 5: this value has no method `set`
  Its type is () -> 'A.{rid : int}.

I've tried to annotate the argument (r:ref or ref:ref({title:string, artist:string, rid:int, initial_uri:string})) but that raised a parse error.