Default positional arguments before required positional arguments throws "not enough arguments" error in callers

[rstoutamore]

Input

→ View on sorbet.run

# typed: true
def function_with_default_arg_first(a='foo', b)
  puts "a: #{a}, b: #{b}"
end

function_with_default_arg_first('bar')

Observed output

editor.rb:6: Not enough arguments provided for method Object#function_with_default_arg_first. Expected: 1..2, got: 1 https://srb.help/7004
     6 |function_with_default_arg_first('bar')
                                        ^^^^^
    editor.rb:2: Object#function_with_default_arg_first defined here
     2 |def function_with_default_arg_first(a='foo', b)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Errors: 1

Expected behavior

Type checking should pass

---

The example is weird code (default positional args should be last), but it's technically valid Ruby (Ruby docs even include a similar example). I ran into this error when adding typing to code that calls a method in a third-party library with a default nil optional argument before required arguments.

The error presented here made investigating particularly tricky, as Expected: 1..2, got: 1 made it seem like it shouldn't be an error in the first place, as 1 satisfies 1..2.

[ipvalverde]

Oh my, this should be a bug in the Ruby language... Interesting how they put it in the documentation:

Describing this in words gets complicated and confusing. I’ll describe it in variables and values instead.