[Bug analyzer/126834] New: false -Wanalyzer-infinite-recursion in readline/bash: sign-flip mutual recursion termination not tracked

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126834

            Bug ID: 126834
           Summary: false -Wanalyzer-infinite-recursion in readline/bash:
                    sign-flip mutual recursion termination not tracked
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: dmalcolm at gcc dot gnu.org
            Blocks: 126830
  Target Milestone: ---

Reduced from readline-8.3 kill.c rl_copy_forward_word/rl_copy_backward_word.

Two functions call each other when count < 0, negating count on each
cross-call.  This mutual recursion always terminates after one step: if count <
0 in copy_forward, it calls copy_backward(-count, key) where -count > 0, so the
guard (count < 0) is false and recursion stops.  The analyzer doesn't track the
sign-flip.

$ cat t.c
extern int do_copy(int count, int direction);
int copy_backward(int count, int key);

int copy_forward(int count, int key)
{
    if (count < 0)
        return copy_backward(-count, key);
    return do_copy(count, 1);
}

int copy_backward(int count, int key)
{
    if (count < 0)
        return copy_forward(-count, key);
    return do_copy(count, -1);
}

$ gcc -O2 -fanalyzer -c t.c
t.c: In function 'copy_forward':
t.c:7:16: warning: infinite recursion [-Wanalyzer-infinite-recursion]
t.c: In function 'copy_backward':
t.c:14:16: warning: infinite recursion [-Wanalyzer-infinite-recursion]

Seen in the August 2026 OpenScanHub mass scan of Fedora 45 (PR 126830)
affecting readline and bash.  Seems to need -O2 or higher.

Trunk: https://godbolt.org/z/srb6dab5x


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126830
[Bug 126830] Tracker bug for -fanalyzer false positives seen in August 2026
OpenScanHub mass scan