[Bug c/89990] New: request warning: Use of out of bound compound initialised struct

[dgilbert at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89990

            Bug ID: 89990
           Summary: request warning: Use of out of bound compound
                    initialised struct
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dgilbert at redhat dot com
  Target Milestone: ---

gcc version 9.0.1 20190312 (Red Hat 9.0.1-0.10) (GCC) 
on Fedora 30, x86-64

We tripped over code like this in qemu  which seems obvious enough that a
warning would be nice:


   int foo(mytype *ptr)
   {
     if (!ptr) {
       ptr = &(mytype) { };
     }

     if  (ptr->p) ....
   }

which worked OK on gcc 8, but seg'd on gcc 9 because ptr->p contained rubbish
since the compiler had noticed that the &() { };  went out of scope at the end
of the if.

So a warning would be nice if:
   a) A pointer is initialised to point to an initialiser like that
   b) That goes out of scope
   c) The pointer is then unconditionally used.

The original qemu code is:
https://git.qemu.org/?p=qemu.git;a=blob;f=tests/libqos/qgraph.c;h=122efc1b7b8413bc69d81f2906c89f06eccd994a;hb=HEAD#l75

  75 static void add_edge(const char *source, const char *dest,
  76                      QOSEdgeType type, QOSGraphEdgeOptions *opts)
  77 {
  78     char *key;
  79     QOSGraphEdgeList *list = g_hash_table_lookup(edge_table, source);
  80 
  81     if (!list) {
  82         list = g_new0(QOSGraphEdgeList, 1);
  83         key = g_strdup(source);
  84         g_hash_table_insert(edge_table, key, list);
  85     }
  86 
  87     if (!opts) {
  88         opts = &(QOSGraphEdgeOptions) { };
  89     }
  90 
  91     QOSGraphEdge *edge = g_new0(QOSGraphEdge, 1);
  92     edge->type = type;
  93     edge->dest = g_strdup(dest);
  94     edge->edge_name = g_strdup(opts->edge_name ?: dest);
  95     edge->arg = g_memdup(opts->arg, opts->size_arg);