[Bug c++/101150] New: null pointer dereference false positive disappears when compiling an additional function

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101150

            Bug ID: 101150
           Summary: null pointer dereference false positive disappears
                    when compiling an additional function
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: adl at gnu dot org
  Target Milestone: ---

Created attachment 51042
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51042&action=edit
source code, plus the three preprocessed versions

I'm observing the following behavior with gcc-snapshot on Debian
unstable as well as when using "x86-64 gcc (trunk)" and "x86-64 gcc
(11.1)" on Godbolt.  This false positive is not reported by gcc 10.

% g++ --version | sed 1q
g++ (Debian 20210527-1) 12.0.0 20210527 (experimental) [master revision
262e75d22c3:7bb6b9b2f47:9d3a953ec4d2695e9a6bfa5f22655e2aea47a973]

% cat foo.cc
#include <vector>

#ifdef FOO
void foo(const std::vector<int>& other)
{
  std::vector<int> v;
  std::size_t sz = other.size();
  v.resize(sz);
  int i = 0;
  for (int o: other)
    v[i++] = o;
}
#endif

#ifdef BAR
void bar(const std::vector<int>& other)
{
  std::vector<int> v;
  unsigned sz = other.size();
  v.resize(sz);
  int i = 0;
  for (int o: other)
    v[i++] = o;
}
#endif

% g++ -O3 -Wnull-dereference -c foo.cc -DBAR
% g++ -O3 -Wnull-dereference -c foo.cc -DFOO -DBAR
% g++ -O3 -Wnull-dereference -c foo.cc -DFOO
In function 'void foo(const std::vector<int>&)':
cc1plus: warning: potential null pointer dereference [-Wnull-dereference]

The two functions differ only by the type of sz, and the warning
occurs only if foo() is compiled but bar() is not.


I *believe* the warning comes from the fact that if sz is 0, the data
pointer of v will still be nullptr after resize(), and that would
render v[i++]=o invalid.  However if sz is 0, the loop will not do
any iteration, so that's a false positive.

However I can't explain
- why changing size_t into unsigned makes the warning go away,
- why compiling the two functions makes the warning go away.

I was expecting the diagnostics about foo() to be independent of the presence
of bar(), and I was expecting to get the same diagnostics for both functions
(preferably none, but I understand it's only a "potential" issue)