${00} should not be allowed

[bram-perl]

Description

In commit 60267e1 (by @demerphq ) for #12948 code was added to disallow $00, $000, ${000}, ...
The perldiag entry added for it:

    Numeric variables with more than one digit may not start with '0'
            (F) The only numeric variable which is allowed to start with a 0 is $0,
            and you mentioned a variable that starts with 0 that has more than one digit.
            You probably want to remove the leading 0, or if the intent was to express a
            variable name in octal you should convert to decimal.

Using blead (b5df4e0):

    $ ./perl -e '$0;'
    $ ./perl -e '${0};'
    $ ./perl -e '$00;'
    Numeric variables with more than one digit may not start with '0' at -e line 1.
    $ ./perl -e '${00};'
    $ ./perl -e '${000};'
    Numeric variables with more than one digit may not start with '0' at -e line 1.
    $ ./perl -e '$000;'
    Numeric variables with more than one digit may not start with '0' at -e line 1.

Behaviour of $00 and ${00} is inconsistent.
I believe the intent was to also disallow ${00}.

Looking at the code there appears to be an off-by-one error;

In toke.c S_scan_ident there are two similar loops to handle this case:

    10113    if (isDIGIT(*s)) { /* handle $0 and $1 $2 and $10 and etc */
    10114        bool is_zero= *s == '0' ? TRUE : FALSE;
    10115        char *digit_start= d;
    10116        *d++ = *s++;
    10117        while (s < PL_bufend && isDIGIT(*s)) {
    10118            if (d >= e)
    10119                Perl_croak(aTHX_ "%s", ident_too_long);
    10120            *d++ = *s++;
    10121        }
    10122        if (is_zero && d - digit_start > 1)
    10123            Perl_croak(aTHX_ ident_var_zero_multi_digit);
    10124    }
    10125    else {  /* See if it is a "normal" identifier */
    10126        parse_ident(&s, &d, e, 1, is_utf8, FALSE, TRUE);
    10127    }
    10128    *d = '\0';

and

    10173        else {
    10174            *d = *s++;
    10175            /* special case to handle ${10}, ${11} the same way we handle ${1} etc */
    10176            if (isDIGIT(*d)) {
    10177                bool is_zero= *d == '0' ? TRUE : FALSE;
    10178                char *digit_start= d;
    10179                while (s < PL_bufend && isDIGIT(*s)) {
    10180                    d++;
    10181                    if (d >= e)
    10182                        Perl_croak(aTHX_ "%s", ident_too_long);
    10183                    *d= *s++;
    10184                }
    10185                if (is_zero && d - digit_start > 1)
    10186                    Perl_croak(aTHX_ ident_var_zero_multi_digit);
    10187            }
    10188            d[1] = '\0';
    10189        }

While the check in both cases is the same (is_zero && d - digit_start > 1) what is different is the handling of d.
In the first loop it always does d++
In the second loop it only does d++ when *s is a digit

Another way to notice this: to null terminate the string the first loop uses: *d = '\0'; while the second loop uses d[1] = '\0';

TLDR: the second check (L10185) should use >= 1 instead of > 1.

Steps to reproduce

$ ./perl -e '${00}'

Expected behavior

Expected error message:

    Numeric variables with more than one digit may not start with '0'

[demerphq]

Note that the second case seems to be designed to handle single character vars ONLY. It only moves one character from *s into *d. I guess i noticed that it actually handles longer variables as well and patched the ascii variant to detect this particular error when I should have noticed that it is firing on variable names longer than one character.

[demerphq]

See #20000

[bram-perl]

Change was merged on blead (c432f9f), closing the issue