What is a mysterious (return_call_ref 6)?

Architecture Hoot: Guile in WASM
1

While introspecting Wasm output (through (dump-wasm) module), I completely do not understand number six in (return_call_ref 6) in bodies function.

My module does not contain Function #6:.

Where I should look for definition of this mysterious 6th funcidx?

For example:

    Function #87:
    Id: $f777
    Type: (type 85)
    Locals: $ret-sp:i32 $v470:(ref 6)
    Body:
      ((block #f
              #<<type-use> idx: #f sig: #<<func-sig> params: () results: ()>>
              ((global.get 447)
               (i32.const 1)
               (i32.sub)
               (local.tee 0)
               (global.set 447)
               (local.get 0)
               (table.get 2)
               (ref.as_non_null)
               (local.set 1)
               (i32.const 0)
               (i32.const 0)
               (ref.i31)
               (i32.const 0)
               (ref.i31)
               (i32.const 0)
               (ref.i31)
               (local.get 1)
               (return_call_ref 6))))
2

I thougth 6 is Type

...
5 ($raw-bitvector): (array (mut i32))
6 ($kvarargs): (func (param $nargs i32) (param $arg0 (ref eq)) (param $arg1 (ref eq)) (param $arg2 (ref eq)) (result))
...

But according to the documentation, retrurn_call_ref accepts funcidx.

3

As far as I can understand, reference to a function holds in $v470:(ref 6), and via

(local.get 1)
(return_call_ref 6)

Wasm gets appropriate reference.

4

This all seems like a bug, I create PR with fix: Update instructions.rst by eduVcJy0UE0cy0zo3BS7XYO3b · Pull Request #548 · WebAssembly/gc · GitHub