Vim9: crash when flattening list of lists

[lacygoill]

Describe the bug

In Vim9 script, flattening a list of lists might make Vim crash.

To Reproduce

Run this shell command:

vim -Nu NONE -S <(cat <<'EOF'
    vim9script
    def Func()
        var ll: list<list<string>> = [['a', 'b', 'c']]
        ll->flattennew()->join()->setline(1)
    enddef
    Func()
EOF
)

Vim crashes.

Expected behavior

Vim doesn't crash. a b c is written on the first line of the buffer.

Environment

• Vim version: 8.2 Included patches: 1-3424
• OS: Ubuntu 20.04.3 LTS
• Terminal: XTerm(353)

Additional context

Regression introduced in patch 8.2.3422.

---

No crash if we don't declare the type of ll, and rely on type inferrence:

vim9script
def Func()
    var ll = [['a', 'b', 'c']]
    ll->flattennew()->join()->setline(1)
enddef
Func()

---

No crash at the script level; but there is still an unexpected type mismatch error:

vim9script
var ll: list<list<string>> = [['a', 'b', 'c']]
ll->flattennew()->join()->setline(1)

E1012: Type mismatch; expected list<string> but got string

---

• backtrace
• asan log