[vim/vim] Vim9: script-local variable can shadow funcref (#7897)

[lacygoill]

Describe the bug

In Vim9 script, a script-local variable can shadow a funcref.

To Reproduce

Run this shell command:

vim -Nu NONE -S <(cat <<'EOF'
    vim9script
    def Func(i: number, v: number): number
        return v * 2
    enddef
    var Func = 0
    var l = [1, 2, 3]
    l->map(Func)
    echo l
EOF
)

No error is raised. [0, 0, 0] is echo'ed.

Expected behavior

An error is raised, because shadowing is disallowed in Vim9 script:

Variables and functions cannot shadow previously defined or imported variables
and functions.

[2, 4, 6] is echo'ed.

Environment

• Vim version: 8.2 Included patches: 1-2549
• OS: Ubuntu 16.04.7 LTS
• Terminal: xterm(366)

Additional context

If we allow a variable to shadow a funcref, it could lead to subtle issues which are hard to debug.

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[lacygoill]

An error is raised

To be more accurate, this line should raise an error:

var Func = 0

Because there is already a function with the name Func. The user should be forced to choose a different name for their variable.

[lacygoill]

Actually, I wonder whether the true issue here is that map() doesn't complain when we pass it a number as a second argument. :h map() suggests that it should be a string of a funcref; not a number.

But even if map() is fixed to check that the type of its second argument is valid, I think it should still be an error to use a variable name which has already been used as a function name.

[lacygoill]

:h map() suggests that it should be a string of a funcref; not a number.

I guess that it doesn't raise any error in Vim script legacy, because the number is automatically coerced into a string. But Vim9 tries to avoid this kind of automatic coercion because they can give unexpected results.

Also, the same issue seems to affect any function accepting a funcref as argument; like filter():

vim9script
def Func(i: number, v: number): number

    return v % 2 == 0
enddef
var Func = 1

var l = [1, 2, 3]

l->filter(Func)
echo l

[1, 2, 3]

[Bram Moolenaar]

Closed #7897 via 6c3843c.