[Bug argparse/28081] New: getopt_long doesn't return error on ambigouos option

["mlichvar at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28081

            Bug ID: 28081
           Summary: getopt_long doesn't return error on ambigouos option
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: argparse
          Assignee: unassigned at sourceware dot org
          Reporter: mlichvar at redhat dot com
  Target Milestone: ---

An option which partially matches multiple getopt long options doesn't produce
an error.

Reproducer:
#include <stdio.h>
#include <getopt.h>

int main(void) {
        const char *argv[] = {"a", "--f", "b", NULL };
        int index, c;

        static struct option long_options[] = {
                {"foobar", required_argument, 0, 0},
                {"foobaz", required_argument, 0, 0},
                {0, 0, 0, 0}
        };

        c = getopt_long(3, (char **)argv, "", long_options, &index);

        printf("c=%d index=%d\n", c, index);
}

Actual result:
c=0 index=0

Expected result:
option '--f' is ambiguous; possibilities: '--foobar' '--foobaz'
c=63 index=0

More information:
It seems this doesn't happen when the two options are different in the
required_argument vs no_argument setting.

Observed on Fedora 34 using glibc-2.33-16.fc34.x86_64.

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