[Bug c/23577] New: suprious warnings about unhanled cases in switches

["gcc-bugzilla at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

gcc generates supruious warnings about switches not handling things which it
could (or should) be easily able to deduce are impossible (e.g. assert(v!=FOO)
just before the switch that gets reported for not handling FOO.

Environment:
System: Linux duncan 2.6.11.10 #34 Thu May 26 11:47:48 BST 2005 i686 GNU/Linux
Architecture: i686

	
host: i686-pc-linux-gnu
build: i686-pc-linux-gnu
target: i686-pc-linux-gnu
configured with: ../gcc/configure --prefix=/usr --enable-shared

How-To-Repeat:
Compile the following fuction with -Wall. This is a contrived example of a case in a
real program (where the fact was due to the impossible cases hitting an unconditional
continue; statement in the first of two switches in a loop).

The same lack of awareness might lead to poorer code, although I do not know the gcc
internals enough to estimate by how much.

BTW a problem also occurs in state machines when you get unitialised variable warnings
because to reach the state that generates the warning you have to pass throigh a state
that sets that variable. That might require some form of state machine detection and
analysis (and might allow gcc to better optimise the state machine too :-)

---begin example program---
typedef enum { FOO, BAR, BAZ } foo_t;

int foo(foo_t v)
{
    switch(v)
    {
    case FOO:
	return 0;

    default:
	break;
    }

    switch(v)
    {
    case BAR:
	break;

    case BAZ:
	break;
   } 
   return 1;
}
---end example program---

-- 
           Summary: suprious warnings about unhanled cases in switches
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dps at simpson dot demon dot co dot uk
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23577