[Bug analyzer/100540] New: -Wanalyzer-file-leak false positive due to conditionals

[andrew at ishiboo dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100540

            Bug ID: 100540
           Summary: -Wanalyzer-file-leak false positive due to
                    conditionals
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: andrew at ishiboo dot com
  Target Milestone: ---

Test program:

#include <stdio.h>
#include <string.h>

char foo(const char *filename) {
    FILE *fp;
    if (!filename || strcmp(filename, "-") == 0) {
        fp = stdin;
    } else {
        fp = fopen(filename, "r");
    }

    char c = fgetc(fp);

    if (fp != stdin) {
        fclose(fp);
    }

    return c;
}

int main(int argc, char **argv) {
    if (argc > 1) {
        char c = foo(argv[1]);
        printf("%c\n", c);
    }
    return 0;
}

False positive:

$ gcc-11 -fanalyzer -c /tmp/test.c  
/tmp/test.c: In function 'foo':
/tmp/test.c:18:12: warning: leak of FILE 'fp' [CWE-775] [-Wanalyzer-file-leak]
   18 |     return c;
      |            ^
  'foo': events 1-6
    |
    |    6 |     if (!filename || strcmp(filename, "-") == 0) {
    |      |        ^
    |      |        |
    |      |        (1) following 'false' branch...
    |......
    |    9 |         fp = fopen(filename, "r");
    |      |              ~~~~~~~~~~~~~~~~~~~~
    |      |              |
    |      |              (2) ...to here
    |      |              (3) opened here
    |......
    |   14 |     if (fp != stdin) {
    |      |        ~
    |      |        |
    |      |        (4) following 'false' branch...
    |......
    |   18 |     return c;
    |      |            ~
    |      |            |
    |      |            (5) ...to here
    |      |            (6) 'fp' leaks here; was opened at (3)
    |


Expected outcome:

The analyzer should understand that without anything modifying stdin, stdout,
stderr, the return of fopen() can not be stdin, stdout, or stderr, so
fclose(fp) must be hit.