They’re planning to support this in the future, but for now what you can do is add an assert after the guard clause.
E.g:
local function x( thing: string | number )
if ( type( thing ) == "number" ) then return end
assert( type( thing ) == "number" ) -- Errors the code if it's a number...
-- but it can never be a number because it returns beforehand.
print( #thing ) -- No more warning!
end