Vim9: cannot refer to prior argument in value of optional argument

[lacygoill]

Describe the bug

In Vim9 script, we cannot refer to a prior argument in the value of an optional argument.

To Reproduce

Run this shell command:

vim -Nu NONE -S <(cat <<'EOF'
    vim9script
    def Okay(mandatory: any, optional = mandatory)
    enddef
    defcompile
EOF
)

E1001 is raised:

E1001: Variable not found: mandatory

Expected behavior

No error is raised, because it is allowed to refer to a prior argument in the value of an option argument, as documented at :help optional-function-argument:

It is possible for later argument defaults to refer to prior arguments,
but not the other way around. They must be prefixed with "a:", as with all
arguments.

Environment

• Vim version: 8.2 Included patches: 1-3081
• OS: Ubuntu 20.04.2 LTS
• Terminal: XTerm(353)

Additional context

Regression introduced in 8.2.2590.

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No issue with a legacy function:

vim9script
function Okay(mandatory, optional = a:mandatory)
endfunction

no error

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Note that this issue is not about asking to be able to refer to a later argument; that's disallowed in legacy. The issue is about the other way around; referring to a prior argument, which is allowed in legacy.

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It would be convenient if it could be made to work again, but if it can't, maybe this limitation should be documented.