[Bug analyzer/95026] New: "leak of FILE" false positive [CWE-775] [-Wanalyzer-file-leak]

["vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95026

            Bug ID: 95026
           Summary: "leak of FILE" false positive [CWE-775]
                    [-Wanalyzer-file-leak]
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

On the following program (obtained after simplifying Mutt's imap/message.c)

struct _IO_FILE;
typedef struct _IO_FILE FILE;
typedef struct _message
{
  FILE *fp;
} MESSAGE;
extern FILE *fopen (const char *__restrict __filename,
                    const char *__restrict __modes);
FILE *f (void);
int imap_fetch_message (int i, MESSAGE *msg, char *p)
{
  if ((msg->fp = i ? 0 : f ()))
    return 0;
  if (p)
    msg->fp = fopen (p, "r");
  return -1;
}

I get:

zira:~> gcc-10 -c -O2 -fanalyzer tst.i
In function ‘imap_fetch_message’:
tst.i:15:13: warning: leak of FILE ‘<unknown>’ [CWE-775] [-Wanalyzer-file-leak]
   15 |     msg->fp = fopen (p, "r");
      |     ~~~~~~~~^~~~~~~~~~~~~~~~
  ‘imap_fetch_message’: events 1-6
    |
    |   12 |   if ((msg->fp = i ? 0 : f ()))
    |      |      ^
    |      |      |
    |      |      (1) following ‘false’ branch...
    |   13 |     return 0;
    |   14 |   if (p)
    |      |      ~
    |      |      |
    |      |      (2) ...to here
    |      |      (3) following ‘true’ branch (when ‘p’ is non-NULL)...
    |   15 |     msg->fp = fopen (p, "r");
    |      |     ~~~~~~~~~~~~~~~~~~~~~~~~
    |      |             | |
    |      |             | (4) ...to here
    |      |             | (5) opened here
    |      |             (6) ‘<unknown>’ leaks here; was opened at (5)
    |

Tested with: gcc-10 (Debian 10.1.0-1) 10.1.0

Note: if I replace the return value -1 by 0, then the warning disappears!