[Bug tree-optimization/96295] New: Wmaybe-uninitialized warning for range operator with empty range struct

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96295

            Bug ID: 96295
           Summary: Wmaybe-uninitialized warning for range operator with
                    empty range struct
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

Consider test-case tui-winsource.c (minimized from gdb sources, filed as gdb PR
https://sourceware.org/bugzilla/show_bug.cgi?id=26282):
...
struct tui_source_window_iterator
{
public:
  typedef tui_source_window_iterator self_type;
  typedef void *value_type;

  explicit tui_source_window_iterator (void *it, void *end) {}

  explicit tui_source_window_iterator (void *it) {}

  bool operator!= (const self_type &other) const { return false; }

  value_type operator* () const { return (value_type)0; }

  self_type &operator++ () { return *this; }
};

struct tui_source_windows
{
  tui_source_window_iterator begin () const
  {
    return tui_source_window_iterator ((void *)0, (void *)0);
  }

  tui_source_window_iterator end () const
  {
    return tui_source_window_iterator ((void*)0);
  }
};

void
foo (void)
{
  for (void *win : tui_source_windows ())
    {
      (void)win;
    }
}
...

With gcc-10, we have:
...
$ g++-10 -x c++  -Wall -O0 -g -c tui-winsource.c
...

And with gcc-11 (g++-11 (SUSE Linux) 11.0.0 20200720 (experimental) [revision
8764e9a3fc43f1117db77d1f056b6c3f15a29db3]):
...
$ g++-11 -x c++  -Wall -O0 -g -c tui-winsource.c
tui-winsource.c: In function ‘void foo()’:
tui-winsource.c:34:40: warning: ‘<unknown>’ may be used uninitialized
[-Wmaybe-uninitialized]
   34 |   for (void *win : tui_source_windows ())
      |                                        ^
tui-winsource.c:20:30: note: by argument 1 of type ‘const tui_source_windows*’
to ‘tui_source_window_iterator tui_source_windows::begin() const’ declared here
   20 |   tui_source_window_iterator begin () const
      |                              ^~~~~
tui-winsource.c:34:40: note: ‘<anonymous>’ declared here
   34 |   for (void *win : tui_source_windows ())
      |                                        ^
...

At gimple, we have:
...
    struct tui_source_windows & __for_range;
    struct tui_source_windows D.2465;

    try
      {
        __for_range = &D.2465;
        tui_source_windows::begin (__for_range);
        tui_source_windows::end (__for_range);
...

So, strictly speaking the warning is correct, because &D.2465 is not
initialized, and consequently __for_range is not initialized when it is passed
as argument to tui_source_windows::begin.

But it shouldn't matter, because struct tui_source_windows is an empty struct.

Workaround is to add:
...

 struct tui_source_windows
 {
+  tui_source_windows () {}

   tui_source_window_iterator begin () const
...
which gives us:
...
    struct tui_source_windows & __for_range;
    struct tui_source_windows D.2469;

    try
      {
        tui_source_windows::tui_source_windows (&D.2469);
        __for_range = &D.2469;
        tui_source_windows::begin (__for_range);
        tui_source_windows::end (__for_range);
...