[Bug c++/86879] New: G++ should warn about redundant tests for null pointers returned from functions with __attribute__((returns_nonnull))

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86879

            Bug ID: 86879
           Summary: G++ should warn about redundant tests for null
                    pointers returned from functions with
                    __attribute__((returns_nonnull))
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: enhancement
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

void* get() __attribute__((returns_nonnull));

int main() { return get() ? 0 : 1; }

This compiles without any diagnostic, but the check for a null pointer is
redundant because of the attribute.

It seems like this would fit under -Wnonnull-compare

Also, this should produce a similar warning:

int& ref();

int main()
{
  return &ref() ? 0 : 1;
}

This check can never fail, because a reference cannot be bound to a null
pointer, but G++ is silent. Clang says:

nonnull.cc:5:11: warning: reference cannot be bound to dereferenced null
pointer in well-defined C++ code; pointer may be assumed to always convert to
true [-Wundefined-bool-conversion]
  return &ref() ? 0 : 1;
          ^~~~~ ~
nonnull.cc:1:6: note: 'ref' returns a reference
int& ref();
     ^
1 warning generated.