[Bug analyzer/105273] -Wanalyzer-use-of-uninitialized-value warns on "missing" default for switch when callers can be statically determined

[dmalcolm at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105273

--- Comment #5 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Similar thing seen in linuxdoom-1.10:

p_floor.c: In function ‘EV_BuildStairs’:
p_floor.c:503:22: warning: use of uninitialized value ‘speed’ [CWE-457]
[-Wanalyzer-use-of-uninitialized-value]
  503 |         floor->speed = speed;
      |         ~~~~~~~~~~~~~^~~~~~~
  ‘EV_BuildStairs’: events 1-9
    |
    |  472 |     fixed_t             speed;
    |      |                         ^~~~~
    |      |                         |
    |      |                         (1) region created on stack here
    |      |                         (2) capacity: 4 bytes
    |......
    |  476 |     while ((secnum = P_FindSectorFromLineTag(line,secnum)) >= 0)
    |      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |                                                            |
    |      |                                                            (3)
following ‘true’ branch (when ‘secnum >= 0’)...
    |  477 |     {
    |  478 |         sec = &sectors[secnum];
    |      |               ~~~~~~~~~~~~~~~~
    |      |               |
    |      |               (4) ...to here
    |......
    |  481 |         if (sec->specialdata)
    |      |            ~             
    |      |            |
    |      |            (5) following ‘false’ branch...
    |......
    |  485 |         rtn = 1;
    |      |         ~~~~~~~          
    |      |             |
    |      |             (6) ...to here
    |......
    |  492 |         switch(type)
    |      |         ~~~~~~           
    |      |         |
    |      |         (7) following ‘default:’ branch...
    |......
    |  503 |         floor->speed = speed;
    |      |         ~~~~~~~~~~~~~~~~~~~~
    |      |                      |
    |      |                      (8) ...to here
    |      |                      (9) use of uninitialized value ‘speed’ here
    |


and also with "stairsize".

In both cases the analyzer considers the case of taking the "default" branch
at:

    |  492 |         switch(type)
    |      |         ~~~~~~           
    |      |         |
    |      |         (7) following ‘default:’ branch...


which would leave this uninitialized, where:

int
EV_BuildStairs
( line_t*       line,
  stair_e       type )

and p_spec.h has:

typedef enum
{
    build8,     // slowly build by 8
    turbo16     // quickly build by 16

} stair_e;

and the only calls to EV_BuildStairs are in other TUs:

p_spec.c: 576:  EV_BuildStairs(line,build8);
p_spec.c: 739:  EV_BuildStairs(line,turbo16);
p_switch.c: 350:        if (EV_BuildStairs(line,build8))
p_switch.c: 488:        if (EV_BuildStairs(line,turbo16))

Probably should assume that a switch on an enum takes one of the enum's values.