Vim9: list slicing doesn't work in a ":def" function

[lacygoill]

Describe the bug

List slicing doesn't work in a :def function.

To Reproduce

Run this shell command:

vim -Nu NONE -S <(cat <<'EOF'
    vim9script
    def Func()
        echom ['aaa', 'bbb', 'ccc', 'ddd'][1]->string()
    enddef
    Func()
EOF
)

Vim prints this message:

''

Now, run this shell command:

vim -Nu NONE -S <(cat <<'EOF'
    vim9script
    def Func()
        echom ['aaa', 'bbb', 'ccc', 'ddd'][1:2]->string()
    enddef
    Func()
EOF
)

E111 is raised:

Error detected while processing function <SNR>1_Func:
line    1:
E111: Missing ']'

Expected behavior

In the first command, Vim prints this message:

'bbb'

In the second command, no error is raised, and Vim prints this message:

['bbb', 'ccc']

Environment

• Vim version: 8.2 Included patches: 1-1127
• OS: Ubuntu 16.04.6 LTS
• Terminal: XTerm(356)

Additional context

List slicing works as expected in a Vim9 script, provided it's used outside a :def function:

vim -Nu NONE -S <(cat <<'EOF'
    vim9script
    echom ['aaa', 'bbb', 'ccc', 'ddd'][1]->string()
EOF
)

" correctly outputs:  'bbb'

vim -Nu NONE -S <(cat <<'EOF'
    vim9script
    echom ['aaa', 'bbb', 'ccc', 'ddd'][1:2]->string()
EOF
)

" correctly outputs:  ['bbb', 'ccc']

[lacygoill]

The first example has been fixed, but not the second one:

vim -Nu NONE -S <(cat <<'EOF'
    vim9script
    def Func()
        echom ['aaa', 'bbb', 'ccc', 'ddd'][1:2]->string()
    enddef
    Func()
EOF
)

That is, we can get one member from a list with something like [123], but not an arbitrary sublist with possibly several members with something like [12:34].

[brammool]

List slicing hasn't been implemented yet.

[lacygoill]

Ah ok, I thought it was a regression, because it worked as expected up to f93c7fe.