Decompilation error for scatter assignment to `$property`

[rdaum]

>@list $mcp:session_for
Verb 9 on #213 -- E_RANGE

>;verbs($mcp)
=> {"create_session", "destroy_session", "initialize_connection", "finalize_connection", "parse_version", "compare_version_range", "compare_version", "unparse_version", "session_for", "user_created", "user_disconnected", "do_out_of_band_command", "package_name", "wait_for_package", "nominate_for_core", "handles_package", "init_for_core"}

>;verb_code($mcp, "session_for")
=> E_INVARG  (E_INVARG)

Either in decompilation, or in verb resolution.

[rdaum]

Error is in decompilation:

2023-09-22T11:50:07.354663Z  WARN ThreadId(16) task_run: crates/kernel/src/vm/bf_verbs.rs:365: verb_code: verb program could not be decompiled object=#213 verb="session_for" error=malformed program: expected assign for optional scatter assignment
2023-09-22T11:50:07.356071602Z 
2023-09-22T11:50:07.356076241Z Stack backtrace:
2023-09-22T11:50:07.356079628Z    0: <core::result::Result<T,F> as core::ops::try_trait::FromResidual<core::result::Result<core::convert::Infallible,E>>>::from_residual
2023-09-22T11:50:07.356083846Z              at /rustc/eb26296b556cef10fb713a38f3d16b9886080f26/library/core/src/result.rs:1961:27
2023-09-22T11:50:07.356087082Z    1: moor_kernel::compiler::decompile::program_to_tree
2023-09-22T11:50:07.356090108Z              at ./crates/kernel/src/compiler/decompile.rs:774:9
2023-09-22T11:50:07.356092963Z    2: moor_kernel::vm::bf_verbs::bf_verb_code::{{closure}}

[rdaum]

Problematic statement: {who, ?package = $nothing} = args;

[rdaum]

Disassemble from our code:

  0: Push(Name(11))     (line 0)
  1: Scatter { nargs: 2, nreq: 1, rest: 3, labels: [Required(Name(18)), Optional(Name(19), Some(Label(0)))], done: Label(1) }
  2: Imm(Label(0))
  3: Imm(Label(1))
  4: GetProp
  5: Put(Name(19))
  6: Pop
  7: Pop
  8: Imm(Label(2))      (line 1)
  9: MakeSingletonList
 10: PushLabel(Label(2))
 11: Catch(Label(2))
 12: Push(Name(18))

Vs LambdaMOO:

   0: 076                   PUSH args
  1: 112 013 002 001 003
     018 000 019 011 018 * SCATTER 2/1/3: who/0 package/11 18
 11: 100 000               PUSH_LITERAL #0
 13: 100 001               PUSH_LITERAL "nothing"
 15: 009                 * GET_PROP
 16: 053                 * PUT package
 17: 111                   POP
 18: 111                   POP
 19: 100 002               PUSH_LITERAL E_PROPNF
 21: 016                 * MAKE_SINGLETON_LIST
 22: 112 002 034           PUSH_LABEL 34

[rdaum]

And generated code of just this expression, outside of the rest of the verb:

[
   Push(Name(11)), 
   Scatter { 
       nargs: 2, nreq: 1, rest: 3, 
       labels: [Required(Name(18)), Optional(Name(19), Some(Label(0)))], 
       done: Label(1) 
   }, 
   Imm(Label(0)), 
   Imm(Label(1)), 
   GetProp, 
   Put(Name(19)), 
   Pop, 
   Pop,
   Done
]

vs LambdaMOO:

  0: 076                   PUSH args
  1: 112 013 002 001 003
     018 000 019 011 018 * SCATTER 2/1/3: who/0 package/11 18
 11: 100 000               PUSH_LITERAL #0
 13: 100 001               PUSH_LITERAL "nothing"
 15: 009                 * GET_PROP
 16: 053                 * PUT package
 17: 111                   POP
 18: 111                   POP

[rdaum]

Bug fixed in 664404a9e63ff9e10b1da16837fe50a86fff0648 Problem was actually with decompilation of all expressions in optional scatter assignments.